Most anything that filters out spam domains doesn't follow shortener links to them. Web forum spam is full of shortener links. Stop making shorteners.

Obfuscating spam links. It is much harder to tell a fake/crappy url when it is hidden through a shortening service.

I don't know what the parent commenter had in mind, but perhaps bad spam filters don't follow redirects and bad link filters don't filter certain TLDs?

Why would a link filter want to filter whole TLDs? Shouldn't this be based at least on the domain name rather than discriminating whole countries by blocking their TLDs?

I don't know. I could imagine really stupid spam filters looking for strings matching /http:\/\/.*\.(com|net|org)/ or something. Of course that's no one else's fault but the filter.

Like other comments have said spammers use it as a form of obfuscation. They even chain URL shorteners to obfuscate things even more.

I work for an anti-spam company. We see a constant stream of spammy short URLs -- peaking at tens of new unique links per second.