I almost voted never, but chose instead option 1, which is close to never. There are rare exceptions - like a forced crash (causing someone to crash on purpose), but if someone crashes or gets a mechanical, or sick, that's not an exception. This is 2017, not 1957. The idea is to win. Winning is what you get paid for. Winning pleases sponsors. If you crash, it's probably your fault. If you get a mechanical, it's probably your team's fault. If you get sick, it's you and your team's fault (or bad luck). If a person runs in front of you slowing you down, too bad. If a train goes by, it sucks, but there's no rule about it, nor should there be. Shjt happens. Welcome to modern competitive professional sports.
Long ago cycling gave up the quaint notion that the yellow jersey should ride most of the day at the very front of the race and dictate who could attack when. The "gentleman's rules" of things like opportunistic attacking should have went with it. Either that, or make it an actual rule.