There's a difference between a runner going out of her way "to make intentional and unnecessary contact with a fielder" and this. There is a tag being applied - the runner didn't have to go out of her way, and contact is going to be made whether the runner wills it or not. There is a difference between malicious and this.
Which softball rule is being broken? If it's illegal then it's illegal, but attempting to dislodge the ball is a normal part of the game, IMO - kind of like chin music: one of those unwritten things. I don't teach it, but I get it.
I guess I'm just old.
This isn't about incidental contact. Per the OP, the 8U coach is teaching the players to try to slap the ball out of the defender's glove on a tag play. IMO, this would fall under the "intentional and unnecessary contact" category. Do you think this is an appropriate thing to teach an 8U player?