I get that a formatter should reject syntactically incorrect code, but I question why it's syntax error in the first place. Coming from using black with python, I generally leave the trailing comma there but for the compiler to care seems weird when there's a standard formatter available.

The compiler cares because they want to enforce this standard (and it may make the parser slightly simpler). That there is a mix, or at least a non-obvious break between what the compiler enforces and what the formatter will accept and produce is an inconsistency in go, I suppose, but not necessarily worse than arbitrary decisions made in other languages.