Get off your high horse. It doesn't cost Comcast anything more for more bandwidth. It's not wireless internet where there's a physical limitation and bandwidth is extremely limited. Comcast was unlimited for a long time and they're starting to roll this BS out to see if they can get away with it..
Uh...yes it does. Much like a big business network, it's all about what bandwidth there is at the "gateway"..and how to manage all of the traffic behind that gateway.
I've worked very closely with 2x smaller ISP.s....as in, "almost weekly, was in their main data center" doing stuff. I saw what it was like at the main data center of ISPs...I myself had quite a bit of bandwidth going through there from sharing clients, and I got exposed to lots of specific details early on when I was building/colocating/managing some globally popular online gaming servers.
An ISP does indeed have a fixed amount of bandwidth, and ISPs oversubscribe their bandwidth, so that it can remain affordable. Do you really think for a minute that if every single client of an ISP was given unlimited bandwidth, and they all maxed out their bandwidth at the same time, and things should still go smoothly and at full speed? Nope!
ISPs have formulas for oversubscription, so that our prices remain tolerable.
Residential broadband is "best effort" bandwidth. It is never guaranteed top priority, high SLA, 100% up and down all the time like business level internet connections. Never...ever. It was never promised to be that either. It's always 'best effort" with a mix of "you can get up to xxx bandwidth". Key word..."up to".
That's one of the reasons that there's a different in price 'tween residential broadband and business internet. Businesses use more resources of an ISP....so of course it has to cost more.