Hello! This is probably going to be long, but I speak from experience!
First, it sounds like you are a wonderful coach! I agree with the other posters' advice of using a sign-up sheet for snacks and clean-up. This is very standard in any activity and it's no secret to parents that if they pitch in with the program, it endears them AND their child to the coach/teacher/director!
As far as the parents, a few words of advice. First, try not to mope about the long hours and work you're doing. YOU signed up to do them and dwelling on them will make you as grouchy as those parents and tempted to stoop to their level.
Next, know that you are correct. You are in line with the program's philosophy and whoever coordinates the overall program should back you up. You never have to worry about what those parents are saying because you are ONLY there to run the softball team. The bleachers are not your problem and in a way, you need to have "tunnel vision" about why you're really there. Only focus on the field and as long as their children are not carrying their negative attitude onto the field, it's not your problem and you should ignore it. In fact, you should be so focused on these snide parents' children that you don't even notice it.
IF their daughters bring the bad attitude onto the field, THEN you have every reason and right to contact the parent. Say something like, "I wanted to let you know of a concern I'm having about Jennifer. She seems upset that she's not getting to play catcher more often. We're just a community team, and I really care about her and want her to have fun. Perhaps she should try switching to a competitive team if she feels we're not providing a quality program for her."
If the parent balks at that idea and wants to help, say, "You COULD help me out by making sure that there's a positive attitude around home. Making snide comments is never a good idea and you could chat with Jennifer about being a good team player."
I have learned that "kill them with kindness" doesn't just get the job done, it actually feels great! I get to keep my sense of dignity and the parent knows that I truly care about their child. Good luck and stay focused on what you signed up for, not what's being said on the bleachers!!