Notice the rules for the NPYLL Spring 2012 season per their web site :

Quote
Procedures - Administration of NPYLL:

•Roster sizes will be limited to 25 players.
•Your roster can have up to 5 players added and subtracted, up to the 3rd game of the season.
•Season will be 7 games with top 4 teams playing for league championship.
•NPYLL will provide awards for the league champion.
•We are following NCAA Rules.
•Rosters may be checked each game by opposing team.
•Illegal players can not be used for games. First time offense, forfeit the game, second time forfeit the season.
•Refs: Home team coordinates refs, both teams pay at game time.
•All players must be members of US Lacrosse.

Anyone notice that rosters "may be checked" for each game by the opposing team? Notice the penalties and sanctions being called out here?