Some bikes do have only one brake but it is at the rear. Applying front brake only at high enough speed or steep enough downhill results the bicycle rolling over the front wheel. So, I would say, simply reckless driving, should be illegal on this basis. Even if there is may be somewhere a very special bicycle safe to ride with the front brake only (have never seen such in all my life but maybe not freewheeling), your bicycle is not.
Also, if you need to adjust your headlight or anything the like, lift the hand that controls the front brake, not rear (usually left).
This happened twice for me alone. One time I was going downhill in the night and decided to adjust the front light with the wrong hand. I was not even intending to reduce the speed, just kept applying the brake because it was downhill.
Second time, more recently, I was adjusting the rear gear regulator on the wheel with wrong hand and suddenly needed to brake. I even was not moving fast, yet happened.
Third time I have observed this happening from the side. The guy was chatting over mobile when he needed to brake. Again, it was not even at significant speed.
You can read the relevant question in Physics stack exchange that deals with physics behind this. Also, another answer on our own site has the same observation: the bike may tip, simple as it is.