Poker websites often have limits on who can enter freerolls. They do this to encourage people to deposit money or reach the VIP club on a website. Sometimes a freeroll limits these tournaments to people who are learning how to play these games.
The most common point is that a site will want to keep a freeroll from being too crowded. A freeroll will be limited to having a specific number of people competing in the event.
You can check the rules on a freeroll to see what works when entering.