Police are free to lie in courts as well, unless there is a specific police officer on trial as the defendant. It might be illegal in theory, but in practice, police get so much protection from police "unions" that they cannot be held accountable for lies they tell in court (again with the exception of a specific officer as the defendant).

I say "unions" because their position of power puts them at odds with every other union in the country. They are not unionized against corporations, they are unionized against citizens. Police "unions" are de facto an arm of the government's will to illegally and violently revoke the rights of the worker class.