They didn't pay for more bandwidth. You ALREADY paid for that bandwidth (say, 5mbit down). Comcast decided they didn't want to provide you 5mbit worth of Netflix, though, without Netflix ALSO paying, even though Netflix had already paid whoever they have as an ISP on their end.
This wasn't Netflix running out of bandwidth and having to increase their uplink speed, and it wasn't the consumer running out of bandwidth and having to pay to increase their download speed. This was Comcast deciding that Netflix was causing you (and your peers) to use too much of your already-paid for bandwidth. Comcast couldn't keep up with the consistent and simultaneous demand on what you supposedly had access to. So, instead, it throttled Netflix (which users saw as being Netflix's problem - hey, Netflix can't keep up!) and then charged Netflix to unthrottle (which users saw as Netflix "buying more bandwidth" so Netflix could keep up). In reality, it was Comcast that essentially oversold their bandwidth (you can have 5mbit down! oh, wait, nevermind, we can't supply all this bandwidth all at once; hey, a lot of it is being used by Netflix, maybe we could get them to pay more so it doesn't look like we were unprepared for demand on services we sold!)
This isn't unlike an airline overselling their flights. The difference is that when a flight fills up and customers who already paid for their tickets can't actually fit on the plane anymore, the airline doesn't start charging the destination more because the destination is using too much space on their plane. They give the customers who can't get on the plane at the very least a free transfer, and I think they get a free future lfight or something, too? Or a refund + flight? Something like that. In other words, the airliner realizes that part of overselling means that you have to deal with the consequences that occasionally come up with overselling... and "deal with" doesn't mean "charge someone else for your own lack of space that you sold as though you had more space than you actually did."
TL;DR: Comcast oversold their bandwidth and decided to make Netflix pay for it.