Goo.gl didn't have customers, it had users. Customers pay, either with money or their personal data, now or the future. Goo.gl did not make any money or have a plan to do so in the future.
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.
The monetary value of the goodwill and mindshare generated by such a free service is hard to calculate, but definitely significant. I wouldn't be surprised if it was more than it costs to run.
Which raises the obvious question -- why make a service that you know will eventually be shut down because of said economics. Especially one that (by design) will render many documents unusable when it is shut down.
While I generally find the "killed by Google" thing insanely short-sighted, this borders on straight-up negligence.
There was a time in Google where anything seemed possible and they loved doing experimental or fun things just for doing them. The ad machine printed money and nobody asked questions. Over time they started turning into a normal corporation with bean counting.
I always figured most of the real value of these url hashing services was as an marketing tracking metric. That is, sort of equivalent to the "share with" widgets provided that conveniently also dump tons of analytics to the services.
I will be honest I was never in an environment that would benefit from link shortening, so I don't really know if any end users actually wanted them (my guess twitter mainly) and always viewed these hashed links with extreme suspicion.
That way they'll make money, and they can fund the service not having to shut down, and there isn't any linkrot.