Question about P2P transfers and account getting locked/investigated

[JOOO]

So I have heard stories about people transfering funds to others and subsequently getting their accounts locked for investigation and sometimes not unlocked.

I know FTP and other sites say they are not financial institutions and do not have the transfer option for this purpose. What purpose then, is the transfer for? And what can one do to avoid getting your acount locked if you want to transfer money to someone?