MaisyPops, but I did start my post by saying that regardless of store type, customers should show manners and treat shop staff politely and with respect. There should definitely be no sense of the staff as a customers slave, to put up with rudeness and poor behaviour.
Customers in middle and high end shops might not out the clothes on hangers, because it is very much part of the job expectation that the shop assistants do it - and sometimes the shop really does want the clothes to be re-hung in a very specific way, and customers doing it actually creates more work for the staff. At the same time, customers can let the staff do this task and be absolutely charming and polite. There doesn't have to be a correspondence between not putting clothes on hangers, or letting the staff being the items into the changing room and behaving in a rude or ungrateful manner.
In all kinds of stores and with all types of customers, there will be some rude and unpleasant ones, but also polite and respectful ones.
Regarding putting things on hangers, again, context is everything. In some stores, you really are treading on the toes of the toes if the shop staff to start trying to do things which are very clearly part of their remit and which they feel a bit uncomfortable about the customer doing (think of it as akin to a house guest getting too involved in helping out with the tidying and cleaning and making the Hist feel uncomfortable) but I agree that in most high street stores, it is polite and helpful to put things back on hangers - I am absolutely not saying it is always okay not to do this, just that things have more shades of grey than just black and white.