I work for a GM dealership, and we offer the same thing. Even in this ad, it reads that it's a replacement part. So you'd have to buy the part, have it installed and then you are covered by the warranty. So it really isn't for free as the warranty is factored in by corporate/dealership management to the price.
At the dealership I work at, we get a lot of people who do go through this, and often times it's on parts that won't really be needing to be replaced again. Also there are of course limitations on the warranty, which means they won't replace it due to wear and tear, and if you're in the rust belt the muffler might not be covered against rust damage etc.