Let's work the math. If 2/3 of the transactions actually processed are "real", then whoever is maintaining the backlog is paying the tx fees for 1/3 of every block. If this is someone with half the mining power then they get half of their third back, so their average cost per block is the fees for 1/6 of the block. If we are talking about someone with half the mining power, their average return per block is 1/2 the fees for a block. Because 1/2 is greater than 1/3, they are making a profit.
I think you made a mistake here by double counting something.
Assume all miners will only mine tx with fee of 0.01BTC, and a full block could hold 900 txs.
Scenario 1 (No one is artificially maintaining a backlog):
If we have 600 real tx per block, the fee will be 6BTC/block
For a miner with 50% of mining power, the expected income from fee is 3BTC/block
Scenario 2 (A miner with 50% of mining power maintains a backlog):
Since we have 600 real tx/block, the 50% miner has to create bogus tx at a rate of 300 tx/block, that costs 3BTC/block
Now the blocks are full and the fee is 9BTC/block, expected income of the 50% miner is 4.5BTC/block.
The net income of the 50% miner is 4.5-3 = 1.5BTC/block. He loses 3 - 1.5 = 1.5BTC/block