Thee is no incentive to include transactions without a fee when solving. However the reference implementation does add transactions smaller than a certain size for free, and since everyone is running this, they do get processed. It would be easy for the major pools to just change this in their code and stop processing transactions,.but the idea behind bitcoin is that network qos is agreed by consensus.

What's special about this mods is that, unlike e.g. just boosting the number of btc you earn per solved block, or lowering the difficulty, not processing transactions is perfectly within the right of a block solver and the blocks wouldn't be rejected by the network.

Will