Wife, children, family, career, access to credit or existing high levels of debt, peer pressure, being seen as the enemy by those who you've sworn to serve and protect, the letter of the law giving cover to actions that violate historic principles, promotions and raises for obedience, bigger budgets, fear of skeletons in your closet being revealed ...
There are a lot of areas of leverage being applied or that can be applied to the police.
If oppressive laws are passed and the courts uphold them we're in big trouble.