What's the idea behind the no-feeding-in-the-last-20-km rule?

Question

I'm trying to understand the reasoning behind the no-feeding in the last 20 km rule. In the Tour de France 2020 Julian Alaphilippe lost his lead due to an infraction of this rule.

Might it be:

1. The competitors are in full sprint. It's too risky to be distracted by using one hand for food/drink.
2. It's unhealthy for the competitors, since for the food to be beneficial, it'll have to be pure glucose. There is too little time left to digest anything more complex, and if they don't get it exactly right, they could "hit the wall".
3. Food in the last stretch starts to act as a deciding factor. A team that discovers a concoction that provides an unfair advantage would win based purely on that food/drink, not the ability of the racers.

I'm sure the reasoning is well established. Is it also well known? More personally, does the rule matter for the amateur? Is there any reason why an amateur should not eat/drink in the last ~20% (if not 20 km) of a trip?

Update:

4. (The following thought emerges from Vladimir's and Weiwen's answers.) Might I have been sloppily confusing the verbs "feeding" and "feeding oneself", and might the rule be more easily expressible in the precision of French's distinction between reflexive and non-reflexive verbs? In other words, it is perfectly allowed to feed oneself ("se nourir") during the last 20 km. What's not allowed is to give (or take) food from someone else ("nourir"). The rule would be clearer if we start to split hairs and ask whether it's allowed for a helper ("domestique") to pass some food to a hungry (and more important) team leader during the last 20 km.

Gratuitous and unrelated comment/question, now that I'm noticing that all the cars clear out during the last 20 km: How do the cyclists tolerate inhaling the exhaust of so many cars during the race until the 20-km mark? It's one thing for a (gasoline) car to pass by; it's another to remain behind it—inhaling the discernibly worse air—for hours on end.

Answer 1

If you do not race in a UCI race you do not care about this rule at all. If you are thirsty, you drink. The end of the trip is where you are at the greatest danger of bonking so if you feel hungry or weak, just eat or get some quick sugar.

Actually, the racers can also do so, but they cannot get new food and new water, they only can use what they have.

UCI Rule 18:
"All feeding (from a car and on foot outside of the feeding zones signposted by the organiser) is strictly forbidden: during the 30 first and last 20 kilometres; ..."

If you are just eating your own food or drinking your own water, you are not acting against any imaginary rule on your trips.

I do not think any of the reasons you list is exactly the right one. The closest one would be number 1, but not literally. They are not in full sprint, but disruptions and distractions have to be prevented at some point. And by default that point is 20 km. Sometimes it is postponed when the weather is hot.

---

Reaction to the update:
I do not think that passing a bottle from a domestique to the leader is forbidden, but I am not sure. It does not appear to be against the wording. I really do think the rule is to prevent the distractions from team cars and from riders moving to the side of the road in the deciding parts of the race. The wording appears to be clearly against external feeding and not against feeding yourself at all.

Answer 2

I think that if you ask around, many people regard many or most of the rules set out by the Union Cycliste International (UCI, or International Cycling Union in English) to be arbitrary. While I don't follow professional racing extremely closely, I also believe that at least some of the rules are not consistently enforced from race to race. For example, this editorial in Cyclingtips by Caley Fretz pointed out that the rule against drafting cars was enforced much too strictly in the 2019 men's under 23 World Championship.

(As a brief aside, cyclists can't simply draft support cars at will for obvious reasons. However if someone sustains a mechanical or crash, it is well-accepted that they may draft the support cars only to get back to the main peloton. Because accidents or mechanical issues can strike anyone, and often not for your own fault, this limited use of drafting is arguably in keeping with the spirit of the sport; if we always enforced the drafting rules to the letter, then potentially a large portion of the Tour de France peloton would have been disqualified by the finish.)

One rationale for the rule was stated in the Cyclingtips article:

The rule is intended to prevent dangerous feeds in the middle of fast chases or leadouts. And to force teams to think ahead, make sure they have enough food and water to get to the end. But there is no real rhyme or reason to the 20 km mark.

The first bullet point in the original question is close to this stated rationale. Interestingly, in 2017, Chris Froome was bonking on a mountain finish. He decided to violate the rule and accept the time penalty. On a mountain finish, a bonk could have cost him the race, so even if you think this is a great rule, Froome's decision seems hard to fault.

The Cyclingtips article also says that the no feeds rule is frequently suspended or altered (this is communicated over race radio, though), and it gives one specific example in 2017.

The rule relates to receiving a resupply from an external party (usually a team soigneur) in the last 20 kilometers (just under 12.5 miles) of a UCI race. In cycling parlance, this is known as taking a feed. Almost all amateur cyclists who race do not do so under UCI rules. You are not likely to be bound by this rule. The question is, can you convince a teammate to station themselves near the finish, would you benefit from taking a feed, and would you have the requisite handling skill to grab the bottle or musette (a bag containing snacks and bottles, subsequently discarded by the rider and left for spectators as a souvenir).

The question could be read like the author thinks that the rule might be that you can't eat or drink within the last 20km. Forgive me if this is obvious: this would be inhumane and senseless, and even the UCI doesn't impose this on its racers. Obviously you must maintain control of your bike, but otherwise eat or drink as you need to.