According to the analysis conducted in this forum thread, Curse's ToU "specifically removes any and all copyright that we as authors have on our addons." Yikes!
There are other bad parts of the ToU language too, including that Curse can change the ToU at any time without notifying authors. Of course, the site is within its rights to impose any ToU it wants on the users, but it's not nice to take control of creations out of authors' hands. I am not a lawyer, so it's altogether possible that I'm interpreting some of this incorrectly. The forum thread, however, claims that lawyers looked at Curse's terms and agreed that the interpretation is correct.
Update: As several of Curse's employees have helpfully pointed out to me, the new ToU are much better. However, I still have reservations about them.