Except that they can, because they are people and people get to have boundaries. There's a difference between "These fans paid to take a pic and get an autograph so I'm gonna smile, be polite and give them a nice memory" and "This fan paid for a photo and and an autograph so they get to lick my face, put my hands on their boobs, grab my boobs, straddle me, jump on my back, kiss me, or whatever else they wanna do and I have to agree."
A photo op is not a free for all. Most fans get that and respect it but there's always some entitled weirdo in the bunch who gets legit heated when told they can't tongue kiss the talent or grab a handful of booty.
I feel like the respect has to be a two way street. Fans should absolutely get what they pay for (a great experience) and talent should be professional enough to provide that but talent shouldn't be expected to forego any and all personal boundaries in respect to their bodies or their privacy and fans should be reasonable enough to understand that.