Those short URLs are often being used in mass mailings, guestbooks, comment forms etc. and they'll get reported pretty quickly to URL blacklists. That gives you the chance to disable the shady short URls before too many people come across them.
Here is a list of URL blacklist providers that you should check each URL against, both before accepting it into your database and again before you deliver it to the user:
You can find out more about this stuff on http://www.surbl.org/
Since I've implemented these checks I only receive complaint emails every other month and no contact from the authorities so far (in contrast to the disposable email services I run).
When you get a complaint about a URL, your software should allow you to disable all the URLs of the reported domain. There are times when the spammers have taken over a forum (or any site really) and created a large amount of spammy content.
Hope this helps.