Maybe it's just me, but I think you're taking this too seriously.
The kids you're talking about are pretty young.
I know some parents who sign their kids up for absolutely everything and the kids get burnt out. I know one mom who signed both of her kids up for anything they even mentioned an interest in and it was a nightmare. The husband put his foot down and said ONE activity per kid at a time. It was insanity.
My son's best friend wanted to play little league baseball. Begged and pleaded. His parents paid the fees and bought his uniform, cleats, etc. After the first week, he was over it. Oh....they made him go to every practice and every game and he was assigned a position. He just sat down on the field. It didn't matter if the ball came right to him. He would not budge to pick it up. When it was his turn to bat, he sat in the dugout.
They could make him go, but they couldn't make him play.
His first baseball season was his last.
It IS a parent's place to teach ethics and responsibility. However, this can be more challenging than it seems sometimes.
My kids were very into sports and loved their teams and were pretty dedicated. Not all kids are like that though.
Not all parents are cut out to be "soccer moms".
With kids the age you are dealing with, I think you should just focus on your kid having fun and not take it so seriously. What other parents pay for and do or don't follow through with should have no bearing on whether your child has a good time or not.
There isn't anybody out scouting 3-5 year olds for professional sports of any kind. The kids who benefit from participating benefit. The ones it doesn't work out for....it's not worth forcing. And it's not worth you worrying about, in my opinion.
You didn't pay for the other little boy's fees, it's not for you to be upset if he's there or not or even wonder why.
Just focus on your kid having a good time.
Best wishes.