The point of the internal affairs department is often to appease voters by looking like they're doing something about police corruption. They're incentivized to

The point of the internal affairs department is often to appease voters by looking like they're doing something about police corruption. They're incentivized to go after high-profile career making cases, not the run-of-the-mill "cop says one thing and the videotape says another". Making it easy and profitable to take clear-cut cases like this to civil court adds incentives to go after these sorts of things.