I agree and disagree
It depends on the remit of the charity shop.
If it's the sort of organisation that is generally viewed as providing a service to the community then you could argue that part their function is to offer bargains to the more skint in the community. People who donate items for sale to these charity shops know this and consider that they're giving to the community as well as the cause. If I have something that I want to give away for this purpose, I'll take it to this sort of shop, or Freecycle/Freegle it.
But if it's an organisation that has a specific remit (to raise money for medical research, for example) then I'm afraid it is their duty to raise money as much money as they can for this cause. Put it this way. People who donate items for sale to these places specifically want the proceeds to go to that cause. It is a donation in kind. If I want to give a tenner to a particular charity and don't have a tenner to spare, but I do have some good-quality clothing that I don't wear, I'll give that to the charity shop. If they could get a fiver for it, I'd be miffed if they flogged it for two quid.
When I volunteered at a charity bookshop, my manager pointed out that a person who donated a book worth, say, twenty quid (something valuable and collectible) would be upset to see their donation discounted. Sometimes the prices the volunteers put on books would be off, but the manager would correct them. And interestingly, most of the people who argued about the prices of the books were rather well-heeled with plummy voices
Oh yes, and we thought that Oxfam's prices were off :D
And just to play devil's advocate: I know one or two (rather irritating) people who will buy things from Oxfam because they view it as more posh than other charity shops. They can't bring themselves to admit that they're charity-shopping because they're skint. So they'll bang on about the unique and fabulous finds. Perhaps there is an element of catering to this market (albeit not one I personally agree with).
By the way, most of my stuff comes from cheap and cheerful charity shops or from eBay