I don't think Twitter considers its API to be proprietary. There have been numerous clones and as far as I know they've never said anything about it.
I am, of course, not a lawyer and am not seeking a legal answer. The question is, if one were to compete with Twitter and if the API is an important part of the competitive product, why wouldn't you seek total interop with Twitter? That way developers are not in any way locked in to either platform.
I'm not telling anyone what to do, or making any moral judgements, just coffee-housing, kibitzing, pontificating, hopefully harmlessly. :-)