They provide this public good without any help from the Government. Public goods can be provided without Government assisstance.
They also provide this public good without any ability to force anyone to pay. They do not wish such an ability.
Street musicians prefer to attract a crowd, even though they know that means they will have more free riders on the day. Free riders are harmless to them at worst, and actually can be positive (they add prestige and may perhaps tell someone about it later).
People fear that public goods cannot be provided because everyone will choose to be a free rider. But this is not what happens. In real life, everyone can be a free rider for street musicians, but people choose not to be. Some pay.
Enough pay that many street musicians go back the next day and do it again. Their actions reveal the pay is adequate and the profession preferable, for them, to any other.
How can it be that a profession can be viable when it provides nothing but a public good and everyone can have a free ride? This directly contradicts mainstream economic thinking on the matter. The only thing to do is conclude that those thinkers are mistaken (and to wonder why they failed to notice such a common place fact).