The major shortening services do check with Google Safe Browsing, SURBL, and other lists of unsafe URLs to prevent use by spammers, phishers, or other bad guys — but these lists are reactive, waiting for reports before disabling links. Besides, new bad sites spring up all the time — and that isn’t going to change any time soon. A web page that was safe yesterday may be infected today, and then cleaned up tomorrow. There’s no way to know, and URL shorteners only add to the problem.