I think that part of the issue is that there never has been a official "definitive" rule set issued by bally, williams, gottlieb, stern and/or jersey jack for any machine that they released.
and for that reason, it's kind of hard to pin down a company and say "hey! this is incomplete, you need to finish the code"
however, if we ever get a point where they explicitly spell out the rules and feature set of a game and then never deliver, then you may have a leg to stand on from a legal standpoint.