Question about parody doujins being legally sold in the US

Dear internet users. I've noticed lately that there has been a lot of parody doujins being sold in English legally. I guess I've been living under a rock, but when has it been made legal for to be sold? I guess I'm just confused on what is happening right now. Nay, flabbergasted since I believe that those parody doujins (ex: Girls and Panzer, Fate series) cannot be sold unless they have permission or right from the original author or company who has the right from them. Thanks for your time. Any answer that can help me on this one is highly appreciated.

That's not necessarily true and there's actually a fair number of parody duchins that are sold
We have some Dragon Ball and fate. It's more an issue of people don't want to publish them. Except for a few examples of self-published artists