Do you think they're lying when they say all positions are open? Let's say a team has a really solid short stop. At tryouts, a girl shows up who's a total short stop rock star. I would assume solid short stop would lose her position to the rock star, maybe get moved to 2B or something. So that even if certain girls are already on the team, they aren't necessarily guaranteed their preferred positions if better talent joins the team.
No, I don't think they're "lying." I think they're hedging their bets, wanting to have all options available to them to form the best possible team. Again, no objection to that. But, if you've already got 8 of 10 players locked in, I think it's kind (notice I didn't say "fair") to let parents know that's the case so that they don't put all their eggs in one basket and will be more likely to look at multiple teams. Just evens the playing field a bit, I guess.