Does it have to be so sinister? Were you a trooper doing speed enforcement, what would you care whether someone give fifty bucks three years ago to your agency's own union, or the FOP? If you are like other union members I've known, you might resent your union for not sticking their neck out for you enough, or for negotiating a weaker contract whatever you had before you unionize, overpromising and underdelivering and so on, the first thing secured during a contract negotiation being automatic dues deductions from your paycheck. Or maybe you like the union, but the "optics" of these stickers is not doing rank and file police any favors, mainly just the often AFL-CIO-affiliated unions themselves.
So Billy BMW clocks in at a brisk but almost reasonable 120mph as he was at least using his turn signals, you chase him down, you "light him up," then you see a union supporter sticker, would that sticker more likely than not work to his advantage with your discretion, because he may have kicked in a few bucks, or by the time you began interacting with this person, wouldn't other things, like his pulling far enough over to minimize your exposure to traffic and so on, matter more? These stickers offer utility not limited to bids for latitude, including political statements, that yes I think the police union, the most vilified institution in the US, is worth defending, and at the risk of getting my car keyed I will stick it on and hope others give money too. Another utility perhaps, maybe it's a signal to cops that, when they pull you over, run your plate and see that you have a CCW permit, that if you concern yourself in any way with police unions, you might be a little more likely not to try to shoot your way out of a ticket as you decide with how much caution you need to approach.
Besides, I routinely see people with these stickers and emblems and thin blue line things driving way too slow. Perhaps they like I do feel that there has been a war on police and it is a thing to push back on, on a flag mast on one's house, in Thanksgiving dinner table arguments, and yes, the bumper of one's car.