The specific issue is that some players have made hacked servers allowing for more players in a single map than the current max limit for the beta. If you choose to play on one of these servers, EA warns that your account could be banned, which will also block access from any other EA games purchased on Origin, the publisher's downloadable games service.
No surprise that EA wants to shut down hacked servers, but these hacked servers can show up in the regular server list, so playing them might be as simple as turning on the game, loading up the server list, and clicking a couple times without even thinking about it. Players don't need to visit sketchy websites or download programs that actually change the game in any way. Getting access to Battlefield and any other game you've purchased from EA's service revoked for such a simple act seems pretty rough punishment.
By John Stanhope