I wonder if there's a comedy kissing etiquette

I'm watching Whose Line, and I noticed that when people kiss - even if it's making out type kissing - they don't move their lips at all. They just press their mouths together and use their other body language to communicate that they're supposed to be making out. At first I thought it was just because the boys didn't want to kiss each other too enthusiastically, but it happens in boy-girl kisses too.

Now I know that in film and TV and theatre when characters are kissing, the actors kiss properly. But I haven't seen that much improv outside of Whose Line and I've never paid much attention to kissing in sketch comedy etc. I wonder if this is some general rule of stage etiquette in improve and/or sketch comedy, or if it's just exclusive to American Whose Line?