Do you guys sit on santa's lap at the mall as well? If not how is it different?
I think the answer is to do with the spirit in which the interactions occur. Santa's legend is that he brings gifts to children at Christmas, so in nearly all cases, as people get older, belief changes and people "grow out" of it. Not to mention that the quality of the experience varies a lot by location. Adults/older teens wanting to meet "Santa" just doesn't fit with the experience.
Now Disney, well Walt himself said that the parks were not just for children, and you only have to look at some of the merchandise and experiences in the parks and resorts to see that Disney today embraces that spirit because they can, through the people they hire and the experiences they create, do something that a lot of adults and a lot of kids love. At the kid level, up to a certain age then they will think it is , for example, "Mickey" whereas adults and older kids "know" that it is not, and never could be, "Mickey" BUT they can believe it is, and that belief, to the degree most people have it, does no harm. Disney makes money. People love the experience. It's the same, basically, as wanting to meet someone famous or a sports star because you are kind of meeting a product than a person, but if the experience is enjoyable then what does it matter as it harms no one.
Earlier in the topic someone was talking about a weird guy getting a picture with Cinderella, then running off to get it framed, then trying to give it to her despite the CM with her trying to get rid of him, and then security having to come. That's obviously someone who wants the experience to be more real than it can be and probably has other issues, putting it mildly. If he just wanted a picture, then that's normal because thats what most adults expect from the character meet and greets and experiences.
I have to admit that on our first visit when we did CRT, and the pictures before the meal, that when it was my turn I did just go and stand next to "Cinderella" and smiled, and "Cinderella" insisted on linking arms for the picture, which did make it a nicer picture and I was cool with that, but I would never have tried to do that without her saying about it first (if that makes sense) because that's my expectation on how the meet and greets work.
I've found all the characters to be very friendly and I'm always happy to just get a nice picture, an autograph sometimes, and I'll make way for the next guest, and I don't in any way begrudge it when younger guests take longer and get more time.
Just my perspective on it and I guess it's the kind of one most adults have on it.