The thing is, context is everything......and most posters on this thread have assumed all contexts are the same and so all behaviours should be the same. Whilst having manners and showing respect for shop staff are always important, exactly what is and isn't the right behaviour regarding things like hangers does differ according to country, type of shop etc.
As has been said, in many countries, customers have the hangers taken off them before they go into the dressing room, and in some countries, it is usual for sales staff to put things back on hangers, collect the items from changing rooms, or various other patterns which might not be seen regularly here.
Likewise, not all items of clothing are bought and tried on in mass chain stores where there are huge numbers of customers trying on vast numbers of things, in changing rooms manned by a stressed teenage girl. In many middle and high end shops it isn't just about the clothes, but staff providing a very personal service - so taking items from customers as they gather them in the shop floor and taking them to the fitting room and hanging them up there,mperhaps undoing zips etc, to ease the customers trying them on, perhaps hovering an differing to get other sizes, giving advice on how items look/suggesting other items to go with it, taking the chosen items to the check out desk, gathering unwanted items from the fitting room and putting on hangers and returning to the shop floor. This personal service might also involve offering drinks or other services such as delivery/holding items for the day and certainly won't involve the staff insisting the customer does anything, never mind putting clothes on hangers. Whilst this might be the more high end luxury shops, the idea that everyone in all shops re-hangs unwanted items, or that it is the height of rudeness not to is simply daft. Context is everything. Yes, in your chain store, it seems dual to make some attempt at re-assembling the items for return to the store, but often button won't be re-done up and a customers attempt at hanging might still need a bit of work from shop staff to make the items as attractive as the shop would wish them to be on display.
And again, Op wasn't especially bothered about hanging the items, it was being told in a rather insisting tone to do it, that really gor her attention. I would think that even in the most basic shop, rudeness from shop staff isn't what people want. It's one thing to hope customers will re-hang the clothes, perhaps have a sign up to politely request it, or to politely ask if someone could do it, but to insist is rude. If the customer doesn't do it through choice or after a polite request, it really is part of the shop assistants job to do it.......and shop assistants need to be trained to know there are some things which it is hoped a customer will/won't do, but it isn't good customer service to insist on, and some things have to be the choice of the customer. I would say re-hanging the clothes is one of those. So yes, it is helpful to do it and yes it is isn't good to thrust a tangled pile of clothes at someone already holding lots of stuff, but if those things happen to you in the service sector, you smile and just get on with sorting it, or ask politely...you don't insist.
When you are working in a shop, you are providing an experience for the customer, not just the clothes. In a sense, you are the host and you are required by your company to be a good host. You hope the customers will behave like polite guests, but if they don't, you still behave as a great host and don't point out their failings to them.
An equivalent might be having lots of friends to stay for a weekend. You'd hope that they might offer to help clear up a meal, or bring some wine or a gift, or they would help in some other way......but if they didn't, you'd just keep smiling and clearing up etc etc. Of course, with friends, you don't need to invite them again if they are a bit rude. With customers though, as a shop assistant, you are being paid to offer a service to those who are helpful and also to those who are less helpful.