The amount that the miner can claim is hardcoded in the client, something like "if block no < X you can create out of thin air only N coins". After some time N

The amount that the miner can claim is hardcoded in the client, something like "if block no < X you can create out of thin air only N coins". After some time N will be zero and the incentive for the miner will come purely from fees. Of course a miner could just use a different, bigger value but then the block would not be accepted by anyone else and be basically worthless. The rules are there because everyone is using the same client or bug to bug compatible alternative.