When I run any process I expect it to restrict all its actions to servicing direct functions related to that process. Plura is expressly unrelated to ANY process/game it is bundled with.
I would personally be annoyed/angry that a program I was using, web-based or not, was using my computer's resources to make money without my knowledge. In fact, you're not just using my computer's resources, you're also drawing more power from my electrical grid, because a processor doing more work consumes more electricity. That's something that directly costs me, the end user, more money. So I would absolutely classify this as theft of service unless the user explicitly agrees to have Plura running in the background.
If there is ample warning to the user, I am fine with this, and would consider it a good idea. Otherwise, it feels really sleazy and wrong.