I will entertain a specific example, and might even side with you on that example if you have one. But I have seen far too much bullshit like "they don't work in summer so fire them" etc.
I gave you a specific example: pay based on merit. This is no go zone for most unions and I see it as a unprofessional and sometimes unethical stance. It is also a work condition that every management employee, including the CEO, has to accept so you can't really argue that there is a double standard.