Any miner that would want to steal a block fee would have to mine two blocks in succession before the rest of the network gets another one. It's possible, but quite unlikely, and if you don't pull it off you've thrown away a lot of hashing cycles. The only way to make it probable is to control a majority of the network hashrate, but in that case, there are much larger problems afoot (i.e. a 51% attack).
It does place an upper bound on the total amount of BTC that can be laundered through transaction fees on a single block, though. I won't do the math right now but it's some multiple of the block reward fee that depends on what percentage of the network hash rate you control.
Fortunately (?) for the would-be launderer, the average pool is mining a good deal more than one block per day, so you just spread your laundering across a series of blocks.
It does place an upper bound on the total amount of BTC that can be laundered through transaction fees on a single block, though. I won't do the math right now but it's some multiple of the block reward fee that depends on what percentage of the network hash rate you control.
Fortunately (?) for the would-be launderer, the average pool is mining a good deal more than one block per day, so you just spread your laundering across a series of blocks.