In middle or high end stores, the staff would NEVER ask the customer to re-hang the clothes.....part of the experience is that the staff do this kind of thing, take the clothes W and hang them I the fitting room, offer to run around collecting extra sizes, be on hand to offer any help and advice needed......and basically to provide a pleasant shopping service. To be honest, whilst some customers might put things on hangers, it would be the expectation of both staff and customers that the staff will do it. And as I say, they would NEVER ask the customer to do it - that would be considered rude and very poor service.
That said, in such shops, there aren't 20 people trying on all the time and taking in 15 items each and piling them up on the desk as they rush away....things will be calmer and more sedate.
In cheaper shops, I think it is the norm to put things back on hangers, but I also think that if customers don't, it isn't great customer service to ask them to and especially to insisit forcibly that they do......there should still be some kind of customer service instinct even in cheaper shops. Yes, it's nice in such shops to help the staff out and yes it's polite, but insisting customers do it verbally is a bit like an adult saying to another adult 'please' in a firm voice,not point out that they haven't used good manners.....not really good manners to do this to a child.
If I were asked to put clothes back on hangers (and generally I do, unless they are very tricky or somehow one has got lost in tryi things on and swapping sizes etc) then I would probably do it, but raise an eyebrow at the being asked, especially if by a young teenage shop assistant, when I am a bit older.
Customers should behave decently - saying thank you to check out operators and changing room staff is simply manners. I think customers can expect some kind of service too though, and not just a surly teenager who isn't actually offering any level of service....it cuts both ways, and in some shops the level of service is woeful.