It might be like https://en.wikipedia.org/wiki/California_Proposition_65_(198... where the incentives are to mindlessly post a warning without any effort to confirm whether it's necessary. Result: warnings on everything until they're nearly unavoidable.
Them's the breaks. When you have such laws like that "everything causes cancer in california" one, the only recourse companies have is to cover their asses.
It takes one guy to file a suit that he got a seizure from your game when it didn't have a warning. Even if he didn't get a seizure, have fun in court.