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.
About Dave Winer
Dave Winer, 54, pioneered the development of weblogs, syndication (RSS), podcasting, outlining, and web content management software; former contributing editor at Wired Magazine, research fellow at Harvard Law School, entrepreneur, and investor in web media companies. A native New Yorker, he received a Master's in Computer Science from the University of Wisconsin, a Bachelor's in Mathematics from Tulane University and currently lives in Berkeley, California.