I think the only reason there may be less unethical bow hunters around is only because there are less bow hunters around. Percentage wise, I would imagine the number of unethical hunters would be close regardless of weapon chosen. Add to that as was stated that the length of a shot being shorter, the opportunity to do stupid stuff is lessened when bow hunting, which drops the number of incidents - but doesn't necessarily mean the ethics level is greater than other hunters.

I must say, though, I think I'd rather get shot by a bullet than some of those arrows...