It's just that every job is designed and paid for differently. I think the company simply has a budget to offer as payment for a certain outcome.

Think about it from the opposite direction: a business owner could state I have never fully understood the insistence that employees get paid when they are using the restroom.

It would be insanity.

I attribute a lot of this to the unreasonable demands from some unions to create job definitions that are impractical ("driver cannot be the person who loads truck"). Fortunately, unions are becoming increasingly irrelevant.