First and foremost, I am 100% against cameras facing the driver in the cab. With that said, let me explain how these systems actually work, or at least the system my company wanted to use before we raised enough of a stink to get the idea shot down.
The camera isn't always recording video or sound. It only kicks in when an event such as a sudden acceleration, deceleration, violent lane departure, etc. triggers it. The recording actually goes to a third party company independent of the trucking company. It's kept on file for a certain amount of time in case the company requests the footage for an accident review. Until that point, nobody at the company views or hears any footage whatsoever.
For those of you worried that someone will be peeking in on you while you're naked, get over yourselves. Nobody wants to see you accidentally, much less intentionally.