Yes, there is a certain logic to it. Except that, it implicitly recognizes that there may be other considerations which may be more important to a worker than receiving a certain amount in pay- which is precisely the possibility which the minimum wage would deny. And while the collective in the form of the union has more power to effect, on the other hand it may not accurately reflect the needs and desire of the individual.
I'm not sympathetic to the union, but what you said here gave me some pause. Ultimately, in a specific instance where a union is able to negotiate certain benefits (as an example, 6 months paid parental leave and 2 months paid annual vacation) they may be able to justify paying slightly below $15/hr. But to be sure, the safest way to protect the general population who probably do not have a good union fighting for them is to mandate a $15/hr minimum wage. It only takes a few bad outcomes to create a situation where workers would be offered food and accommodation to make up for lost pay.
