From my experience, they've always assumed that I want to put it on myself. Even after I say I'd like to try it on or elude to it, they still don't assume that you want them to do it for you, one time it seemed that the girl assumed I had a 129 brush in my back pocket! Not really, but you get the hint... It's like Blindpassion said, they can't read your mind, and I think for the most part they understand customers can get overwhelmed with the product, so they don't force it on you. But generally they don’t mind when you know what you’re doing, so I just help myself. Just yesterday I was at a mac counter looking for a lipstick and the girl even handed me the sanitizing beaker, and I went to town!
However, and I know this may be getting off topic, my only stipulation is when I want to swatch eyeshadows and blushes as I understand it’s unsanitary. I’ve seen threads where people complain about people putting their fingers in the eyeshadows, BUT a lot MA’s do it themselves and have no problem with you doing it either but I tend to ‘read’ the MA’s first, as to avoid overstepping my boundaries.