They do it because all of the stores have to have the exact same clothing and accessories on the mannequins.
If someone has to resort to buying something that's on a mannequin (who require specific sizes), that means it's the only one and there's not another one left in the store in the mannequin's size. Thus, the mannequin would not be wearing the correct attire. There's also the issue of the clothes getting damaged from the pins, tape, etc that are sometimes used for show, and that's the reason they'll often give if you demand an explanation, but it really just comes down to each mannequin having a very specific uniform.
I've seen several pictures of Cameron in Abercrombie attire, so I'm sure she's familiar with this rule, and she probably just expected them to bow down to her greasy ass and bend it for her because she's a starrrrr. Then, she was so appalled when someone !!gasp!! didn't treat her like she was better than everyone else that she started letting loose with nonsensical insults.