Theymos made an incorrect assumption, hash rate is in no way affected by the inclusion or non-inclusion of transactions, there is no advantage to not including transactions. There may have been a very small bit of CPU used in the CPU solo mining days when creating a new merkle tree when a new transaction was received, but these days where multiple pools have over 4TH, the number of included transactions in no way impacts the hashing that miners are doing - they don't even know how many transactions are in the block data that is being hashed.
Secondly, even if 50% of the blocks included no transactions, Bitcoin would keep on working. There was already a "mystery miner" that was using a botnet that didn't include transactions, and his 10% of the hashrate was merely a curiosity.
If a bad actor has more than 50% of the hashrate required to deny transaction inclusion on more than half the blocks mined, there is a much bigger problem, as they already have enough hashrate to do a 51% attack and can cause more problems by rewriting block history, double spending and erasing blocks. Bitcoin relies on a majority of mining being good, there is little defense against a majority hashrate attack.
Finally there is no motivation for this. If a pool operator was doing something against the interests of Bitcoin, it would be known and miners would leave. Not including transactions would be passing up considerable earnings.