Why is it evil? If we assume that a free URL shortener is a good thing, and that shutting one down is a bad thing, and given that every link shortener will have costs (not just the servers -- constant moderation needs, as scammers and worse use them) and no revenue. The only possible outcome is for them all to eventually shut down, causing unrecoverable linkrot.
Given those options, an ad seems like a trivial annoyance to anyone who very much needs a very old link to work. Anyone who still has the ability to update their pages can always update their links.