Employees trade their hours for the company's money. If you want to trade deliverables for money, you have to be a contractor on a deliverable-based contract. It's a lot riskier, which is why it pays more... sometimes. If you are in a full-time job, and not putting in full-time hours, you're defrauding your employer.
What exactly an employer is paying for with a salaried employee is fairly nuanced and specific to the work. "They're paying for your time" isn't wrong but it's insufficient to explain a lot of the real dynamics there.
Specifically that would make every time you look away from the computer a privilege purely at the discretion of your employer. Not a condition many of us would accept given other options. So it seems to me either this isn't the pure moral reality of the transaction, or we're all transgressors here.
Sure, I'll agree with that, up to a point. And I think, for me, that point is "would you behave differently if your employer had full knowledge of your actions."
Ultimately, people who work two simultaneous jobs are abusing the trust of their employers, and if it becomes commonplace, it's just going to hurt the rest of us. Instead of trusting employees to do the right thing, employers will put draconian employee tracking measures in place.
And why would my employer ever have full knowledge of my actions during the workday? Even in the hypothetical it's extremely invasive. If I need to pray five times a day several of those are landing during the workday. Fuck a boss who thinks they need to know about that.