It wasn't just all the youtube access which caused your slowdown, I'm sure. I would be checking their systems for downloaded stuff as well. Because that's the reason they gave you for bottlenecking, basically, was that you exceeded what they (your ISP) considered fair bandwidth usage. Depending on the ISP, that usage doesn't have to be a whole lot, in reality.
It's why I have to pay extra to ensure I ALWAYS have good connection and don't get bottlenecked. With the ftp stuff I do, and various other things, were I on a normal connection, that would happen to me too.