Holy crap those are some insane dues! I had a similar experience working for a grocery store at that age. I am pro-union since I have seen the good it does for trades, but it seemed scummy trying to convince me to join the union as a kid who was on his parents insurance at the time and didn't need benefits.
Funny enough, the person who replied to you was wrong. I didn't get health insurance. They were dues. The benefits I mentioned were guaranteed raises that I knew I wouldn't get because I didn't plan to work there for more than a summer.
Then why try to convince me to join if I'm paying into insurance and strike funds that I will never see any benefit from? I understand collective bargaining but it seems like they just wanted to trick high school kids into giving away 15% of their paychecks. Also even though the store had high union membership, my managers still pulled crap like calling me to the register during my legally mandated 15 minute break.