Say a group miners (miner mafia), themselves send thousands of microtransactions without fees.  The miner mafia themselves if they win the btc, first accept only transactions with fees then only will accept their own internal microtransactions into the blockchain. 

If the miner mafia has a significant hashing %, do they not significantly discourage users from neglecting fees?  Is there  a reason that every miner group should not adopt this strategy to maximize their profits?