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.