Imagine how for every one user doing this deliberately there are nine who pause a video and forget it in the background, wasting bandwidth in the process.
Like others mentioned - yes, I mean the bandwidth from the perspective of the one providing the service. For the same bandwidth that someone watched 10% of a video, paused it and never watched the remaining 90%, you can show those same 90% to someone else who'd actually watch it. That's without counting the small overheads here and there, but hopefully you get the idea.