Keep in mind I am speaking from experience solely in Texas and unions may be different in the north.
I have been a card carrying member of the Texas Fraternal Order of Police for the past 5 years and was part of the Texas Municipal Police Association for 12 years prior to that. Technically, both are associations, but good luck figuring out the difference between unions and associations (unions elect their officers, association members elect theirs). As a police officer, it's highly advisable to be a part of a union/association because of the legal coverage they provide. I myself have had to use it when I was sued in state district court several years ago. Well worth the money. They also go to bat for our interests in Austin. An example would be that we've begged for years for a law requiring motorists to vacate the lane next to the shoulder when we are on traffic stops. The unions made that happen for us.
Our unions do not barter for our salaries or compensation structures. Those are determined by the cities/county/state where we work.
From speaking with firefighters, their unions are very similar in structure and purpose.
Sooooo, when the FOP or a firefighter's union calls for donations, realize that they're not the enemy. We use those funds to help sick and fallen officers and their families. We don't use the funds to march on the state capitol to better our benefits.
Just wanted to toss this into the ring to clear up a misperception I saw earlier.