Hi Ehren,
You're exactly right - under most circumstances, you have no obligation to remove links. Exceptions may exist for copyright infringement - courts are still working on settling that.
You should almost always nofollow UGC, unless you're editing all comments manually to make sure each link adds value. If you add nofollow, you can basically have a canned response telling webmasters that these are not the links they're looking for. 
