I'm not sure how Germany organizes their rail systems, but I'm guessing the rail companies profit from increased property values like in much of the world. This makes free public transport potentially workable (and even profitable) at least in the subway aspect. The US does not exercise a system like this in the vast majority of cases.

Of course, there's also the fact that if you have a well-designed system, free transport should encourage a lot more economic activity that'll pay back into the funding of the system. That's another thing many cities in the US lack.