[nisa]

Maybe I'm naive but I'd thought along the line that they could issue a warning that he needs a visa to enter and will unlikely be granted access without it. If he gives false information in the visa that process is probably the only option left - but in my opinion this still does not justify the treatment.

[anigbrowl]

While I don't agree with the immigration rules, they are easily available and clearly spelled out on US embassy websites.

[MichaelGG]

Perhaps I don't understand what you're proposing? What would they warn people? He had to go through ESTA and within 2 clicks from the US Embassy Germany's page on that, it says if you're being paid for performing you're not applicable.