If this were the one and only occasion, sure it was a bit much for the guy to lose his job.
But I sincerely doubt it was the one and only occasion. I have no doubt the guy was taking advantage of youthful guests or anyone else who looked like they were an easy mark. Since you didn't have access to the hotel's deliberations about that, you would never know.
It is far more likely that it was hardly the first time. A first time is not going to get a firing. Especially if they are protected by a union.
Chances are more likely that this was the hotel's last straw. The guy clearly didn't think you would complain. That's his problem. He made a big mistake on top of a whole lot of other big mistakes.