I replied back to one of these and explained that due to their anti-union actions, and they way they treat employees, I would never be interested in working there, no matter the salary. I thankfully have never received any more of their spam.
None of this is a reflection of the value of unions from the employees' perspective. But it's important to realize that unions are one of the instances where the employees' and employers' interests are not aligned. So, emailing a recruiter and mentioning unions has only one possible outcome - none.