Its also to cover there bases. If they don't offer it someone somewhere WILL bitch about it. But if someones throwing a fit at customer service that they broke there PS3 because of a firmware update, the Best Buy employee can just say 'Well we offered you the service to do it for you, which you declined.' Same thing with insurances and their Black Tie Protection.
Also, god forbid, some people don't have internet. Now the chance that thats the reason someone has to have their PS3 updated is pretty slim, but it IS a possibility.
Sure, 30 bucks might be a little steep, but you need to set a price somewhere. Best Buy makes a majority of their money on services. They make maybe, 5 bucks on a PS3 sale, if that. I know when I worked there I got no discount on purchasing systems, its the same way with PCs. They have to make there money some how, and games aren't exactly marked up either. Its the accessories to an extent, especially cables, but mostly the services where they make their money.
Like Tristam said, if you're spending upwards of 300+ dollars, you should do some research. And honestly, how many grandmas are purchasing gaming systems for their grand kids. I think the most I got from my grandparents for Christmas was a shirt and maybe 20 bucks...