That's just a membership and paying a cut to the unions. By far the exception rather than the rule.
I totally agree...there really is no professional certification other than the ability to pay dues. The part of the process that mimics certification is the hiring process. The path to the union may be getting picked up by the local symphony...so you you have to pass an audition. But this is not a professional level certification, it's a job level certification which is bit different. I do believe unions develop a profile of their members that crudely documents your skills...such as if you read music or play by ear.
As I said earlier..."musician" doesn't really tick a lot of the boxes to fulfill the academic definition of "professional." I still think we should all aspire to be professional if we intend to play for pay.