Maybe it's just me, but I'm a little confused by the way you phrased your question in your post. But if I do read it right, you're questioning why we ask complete strangers if the person we're making a quilt for would like the fabrics we're picking out or the pattern we're considering. You're also saying how could one stranger (the one we're asking in the store) possibly know what another stranger to them (the person we're making the quilt for) would like or enjoy.
Assuming I've got the question right, I would say that often those who do this aren't looking for someone to make choices for them, they're looking for validation that they have already made good choices in the fabrics/pattern they have already chosen.
I've personally never done this, but I have had folks ask my opinion on fabrics, colors, patterns, etc. in Joann's from time to time. Mostly, they are brand new to quilting and just need some direction and a little confidence boost that they're on the right track.
When I'm making a quilt for someone, I put a lot of time and thought into their personalities, color schemes in their homes, what colors they often wear themselves, etc. I try to always make quilts in hues that they will like, even if it's not particularly my taste.