Though I don't know the real reason, I think the
mangakas are actually flattered when groups or "circles" make their own stories based on the characters. The reason being, is because: since the original
manga is so insanely popular, many people want to create their own stories based off the characters to pay homage to the
mangaka. So, that's a form of flattery and the artists don't mind. Yes, there are some very horrible
doujinshis, but the circles or group of artists put their heart into making it because they enjoyed it so much, so it sells. Another reason why it sells is because
doujinshis are a way to see the impossible. For example:
- To see the characters in new situations and events
- To see two characters whom you wish would end up together, but they don't, end up together
- To create
hentai
And while these may be bad reasons (though they are the main ones)
doujinshis are very popular because of it. The original
mangakas don't mind
hentai situations using their characters, since
hentai is very common and normal.
Doujinshis are harmless because it's only group of fans who wants to pay homage. There is not going to be anyone who will try to rip off the original drawings, since the
doujinshis and the original have very different drawing styles. Everyone knows the difference between the real thing and the fake one, so even if one of the "circles" try to claim it as their own, it's not going to work. Another thing: I believe that even some of the
mangakas make their own
doujinshis off their own characters. For some reasons: a lot of fans complain of no sexual situations in the original, so the
mangakas decide to make
doujinshis to satisfy their fans.
I personally don't think
doujinshis are bad at all. Actually, I would've like to purchase some myself, especially on
Cardcaptor Sakura because *SPOILERS* I really wanted Sakura and Yukito to end up together, but since he's in love with her bro.....

. I think there may be some non-
hentai doujinshis on Sakura and Yukito, but I haven't checked. I hope my explaination answers some of your question, Hylas.