CACO3GIRL,
"Unacceptable" is probably too strong a word. "Inappropriate" or "presumptuous" might fit better.
I don't think a parent should encourage or expect the player to renege on his commitment, much less pressure him to do so. I don't think the parent should fight a turf battle with the coach, putting the player in the middle. I think a player should know that he is not to walk into the coach's office and say how things will be; at most, he may go there to present a request for advance permission. And I would suggest that the coach should respond by appreciating the position the parents have put the player into, and suggesting that the player find a third path.
I would add that a Legion team, like a travel/exposure team, has a very short season. Any questions about the team's coaches' attitudes about missing practices or games should be inquired about prior to making the commitment to the roster. Committing first, getting into the season, and then telling the coach you've decided not to be there one day is not appropriate. Your options are to discuss arrangements in advance, or to accept that if the team won't accommodate you, your choices might be reduced down to either playing for that team and living with the restrictions or finding another team that will accommodate your priorities.
There are, after all, literally dozens of team options out there, and they all have different policies. Find one that suits you; don't ask the one you choose to change mid-season to suit you. If every parent did that, the team could not function.