Mining pools cannot arbitrarily modify bitcoin's policies (that is, they cannot execute a 50% attack) because each pool is comprised of tens of thousands of individual miners running third-party, open-source mining software. Miners connect to the pool just by passing a username and password on the CLI/config file. You can be connected to multiple pools at once, executing the same binary to produce results for each pool.

Bitcoiners are savvy enough to get raised eyebrows if a major pool started encouraging and/or requiring users to use a proprietary, closed mining software that may attempt to modify the bitcoin network's behavior to do something unsavory.