The crux is in the expectation: they expect that they will make X in a given day, but an expectation is not a guarantee or a right. They also could have made X in a given day, but sometimes that just doesn't happen due to various factors.

I'm not sure to whom you are referring when you say "thief", but I was under the impression that we were talking about a person watching, but not re-uploading, a Youtube video without viewing ads. This person does not have Y dollars in his account as a result of copyright infringement since they simply watched the video and did not put it up for further distribution.