Why do the cups in films never have lids?

It's just something I've always noticed. In American Pie, while the guys are in their favorite restaurant, none of their styrofoam cups have lids. In Superbad, the slushie drink Evan (Michael Cera) buys at the convenient store doesn't have a lid. I can't think of anymore movies off the top of my head, but I know there are more. Is it an environmental thing? Is it a copyright thing? Or am I just looking into this too much? lol