Oh, I'm going to disappoint you here. A lot of merchandise with messages, not just these, on it is bought as gifts by well-meaning friends. (Not the MAGA stuff, we see people wearing it all the time.)

People buy gifts for their friends and relatives without really knowing that much about them. "John hates Trump, we'll get him this hat!" John says thanks, puts it in a drawer and never thinks about it again.

As for the Amazon profits from Mein Kampf (as an aside, it's pretty astonishing that this incoherent rant is one of the world's most popular books, but no one lost money underestimating the taste of the world public), I hear those profits go to the National Bocialist Party: https://www.youtube.com/watch?v=vlmGknvr_Pg

(There's about a minute before the penny drops, stick with it.)

As a personal anecdote, I had a friend many years ago who had lost a huge swath of his extended family to the Holocaust. He mentioned that the above sketch was his favorite comedy sketch ever, and when I expressed surprise he said, "It's not just that it's very funny, but it's proof we completely won."

(And it is very, very funny.)