You're right. That doesn't mean that some shops won't ask and expect an answer though. So even though you're legally not required to do so, you'll have to go through an argument about that with the shop, which is annoying and time consuming.
This is not a thing. At least in Germany I never once had anyone ask me why I want to return before I could. 90% of the time you already have a return label in the package. Worse case you don’t and you cannot even request one through the website, you send an email and get the label.
I was only ever asked about my return _after_ it was issued by Charles Tyrwhitt after I ordered and returned a good number of items (due to their fit being all over the place). That was fair IMO.
On the other hand Amazon has numerous times shown that if you return too much (or don’t behave due to some other opaque metric) your account can get suspended. Even my account was suspended once, though I am only a moderate shopper and have returned less than 10 items over my 18 years membership.
I raised the issue with support and asked why (there weren’t even any returns around that time) and they could not tell me but reinstated my account.
Good demonstration of power and a reminder to move away from using kindles.
Amazon returns may be easy until you cross some threshold and end up on their bad side.