I contemplated the topic, visited a local XC/Enduro* race, asked the guys that organise it.
"Are riders without subscription and numbers hindering the race, they are not one or two but about 20%?"
"No. Why would they?"
My conclusion is that any cyclist without registration is no different than any other bystander. Consequently, if the race organisers have taken strain to restrict outside personnel**, then Mσᶎ's answer applies with full strength. On the other hand, if travel is allowed, cycling is no more immoral than walking on the race course. However:
- taking food or water constitutes theft, just as if a random hiker helped himself
- (optional) as unnumbered riders are not in a hurry for anywhere, they must help any racer in need.
- as Criggie explains not subscribing for insurance and for the race does not help the race to be held the next year or the medical crews to be funded.
Those are supposed to be objective arguments, on the basis of which each individual can decide.
* - 100m positive attitude gain, 1700m negative attitude gain, mix of loose gravel road and singletrack of moist sharp stones; hikers, children, dogs, automobiles everywhere
** - for example, I would consider a large warning sign a suitable restriction for hikers, or a police car a suitable restriction to automobiles