"I've never had a client expect to be able to look over my shoulder"
Right well UpWork isn't your client. I don't think your clients should, and to another person's point, yes your clients should trust you.
But this is in the case of when you do the work and the client _doesn't_ pay. This is UpWork covering your ass, when they really have no obligation to. They want a way to prove that you are least did some work, so likely they can go after the client later and recover costs.
This seems absolutely reasonable and honestly a heck of a bargain. Again, I think the implementation of counting keystrokes seems like a stupid way to do it, but coming up with a tracking method to _optionally_ guarantee payment even in the case of client non-payment is the deal of the century in the freelance world. Don't believe me? Try freelancing for a while and see how many clients stiff you after you've completed the job. There are literally advertisements in the NYC subway about it.
(Yes, retainers, payment up front, et ceteta -- no the world does not always work that way.)