Well, I've been to Mexico twice, and in both cases I've entered legally on foot and in neither case did I have to present ID.

I'm just pointing out that the idea that you need ID to legally leave the country is a fallacy. The problem is that when you leave this country, you are entering another country, which generally requires documentation to do so. That is not the fault of the United States government, as such.

However, this is not universally true as anyone who's crossed into Mexico knows. If you want to return to the US, you will need ID. But the United States is not a prison. You are free to leave at anytime...no ID required.