I believe this was put in place because FJ is controlling the players that just use the platform as a "wallet" of sorts. Because FJ doesn't really charge players for transaction/miners fee (which is extremely awesome!), I think having a wagering requirement of at least x2 (which is actually easy to do if you are really gambling) is necessary so that users cannot abuse the free withdrawals.

Not completely sure about my answer, because it is just a theory. Would love to hear why they really placed it.