I'm curious about the opposite -- I'm assuming it likely keeps dead weight around longer too; someone who isn't happy or motivated, waiting around for a few months for the 5k bonus for leaving.

This might actually be desirable for amazon as it likely helps them keep their employees through the rough holiday season where they probably see a larger than average quit rate. I'd be willing to bet that the choice to quit and receive compensation comes shortly after the holiday season ends, thus dangling a carrot in front of the employees to muscle through.

Really good point -- makes a lot of sense

For a company like Amazon a few months probably isn't significant in the grand scheme of things. They don't operate like a startup that is constantly a few months away from failure. Plus they have a lot more employees to fill in the gaps.