Actually, it is untrue that you can just set up and play on any public street in the United States. Especially for money.
Nah, it is protected by the first amendment, as I said. Anyone saying otherwise is going against the constitution.
In Chicago they have that nonsense that you have to get a permit, ha! Unconstitutional, but expected from that state.
Funny thing is that I never asked for money (honestly, as a busker, you will never get that money back from what you invested in the instrument). I was doing it for the fun and social interaction, and it was quite rewarding having hot girls in swimsuits all around me, asking if they could try to play something with the accordion on me

.
Good times.
In many if not most localities, at least in the big cities--yes, you absolutely must get a license. It is usually a bureaucratic formality and a nominal fee, but can be difficult when the street or walkway you desire has a lot of foot traffic and potential "paying audience" members--such as a "hot" shopping or restaurant district, or outside a desirable museum or auditorium or other tourist/sightseer site. Those spots are highly desired by buskers and if they're all taken you have to wait your turn to be authorized. If you do not have the license you can be cited and told to pack up and move along.
I heard ridiculous stories of business owners sending the cops against buskers with the excuse that they needed a permit, restrooms, insurance, and a lot of more nonsense if they wanted to keep playing at the streets! Ha ha.
Truth is that they were just jealous, somehow, that there are live musicians around them when they are paying for (often awful) prerecorded music to be played inside their business.
Of course, later on, they were all turned down when a non profit organization heard about this and initiated lawsuits against the city hall due to them violating the first amendment and freedom of artistic speech.
They all backed up quite quickly, and busking became a protected right in those areas, as how it always was.
This is also true in most of the big-city subway systems--in NYC you definitely must be registered and authorized to play in the underground.
I will never do that. It will totally destroy the point of busking by taking the simpleness of it out. If you can't do it casual, is not worth it.
Unless, of course, you are a professional musician without a job.