Not condoning field loads but other than the fact that most field loads use shot size smaller than #7 1/2 it's all about control . If a club has enough property to keep the shot in field loads on the property and you are shooting for fun what has field ammo to do with safety? I follow the rules with ammo at my clubs but I often wonder if you get shot with a duck load will it kill you anymore dead than a trap load. I reload my clay loads often with field load markings and put them in the dumpster after my shoot and anyone that can afford Bismuth for clays has a hell of lot more money than me. Gun handling safety is the most important factor I have more than once seen a shooter step off of a clays stand with a loaded gun what has shot size got to do with that carless move ?
There is a safety issue in that the sporting clays field is laid out , so that #7-1/2 shot won't travel far enough to reach shooters at other stations. If some fool uses #2 or #4 shot, it carries much farther, and can strike shooters on another station.