Why is public WiFi in Europe so painful?

They made the owner of wi-fi networks liable for all actions. In Spain I think. Mostly for the benefit of the carriers. Because almost everywhere there were open wifi-s before the law. But you can get 1GB 3G traffic for the modest 20 euro .. .

Depends strongly on the country. It's not too bad in some. At Baresso coffee shops in Denmark you just click through to the wifi with no signup/password. And Helsinki has free no-login wifi across the whole city center.

As _delirium said, depends on the country. Sofia (BG) has wifi all over the place while my own hometown Zagreb (CRO) doesn't...

It is liability. In Germany for example they are currently debating of easing up on it, so free WiFi can be deployed in Berlin.