My company will fire you for average performance. They do it quite pointedly. One day you're at your desk, and an HR rep with an armed security guard will come and get you. They'll escort you to an office, explain that you're being terminated, and remind you that you're an at-will employee. They'll give you some forms about insurance, and then the guard will walk you to the door. They do not let you clean out your desk. They not let you say goodbye to anyone. Sadly, this is the same procedure that my company uses if you give notice. They do not except two weeks notice. The same day you give notice, or perhaps a day or two later, you will go through this ritual.
To your point, I work for a sales organization. It doesn't matter if your performance reviews are outstanding. If you go more than a couple of weeks without results, the director of sales operations begin sending you nasty emails. Within a month after that, your escorted out the door.
This might be just the sales world, but at some point people deserve to be treated like human beings. Not like replaceable cattle.