You're right, copyright does not protect facts _in the United States_. Rules differ around the world. In the EU, for instance, there's a "sweat of the brow" criteria for database protection.
Even in the US, though, copyright law is largely irrelevant. When you sign up to use a web service you generally accept terms and conditions that lay out exactly how you're allowed to use the service and the data you consume. From that point on your interactions with the service are regulated by those terms under contract, tort, and other common law statutes. If the terms say you're not allowed to redistribute the data then doing so would be violation of contract and you would be liable for damages. Basically, things get really messy really fast.
I'm an engineer, not a lawyer. So, honestly, I'm not an expert on this stuff. But I think most engineers agree that the current situation is sort of stupid. Philosophically I agree with the copyright law as it stands. Facts should be free. So CC0ing the data is just an explicit, formal, contractual statement that we're not limiting how our data can be used anywhere in the world.
I have started to look into this as well. But it is not as simple, every country in the EU actually has different copyright laws. Also the situation in the US does not seem to be that clear either.
Right, but I am not sure how a POI database has any sense of copyright-ability. Feist v. Rural states that a collection has to have a minimal requirement of creativity.
Foursquare and its venue classifications are probably can be argued for, but strict POIs are probably not.
All in all, I have to say thank you to SimpleGeo though. This is a huge step in the right direction.
http://www.copyright.gov/help/faq/faq-general.html
(But it's very nice of them to make this collection and make it available to all!)