If the expectation is outlined at the beginning as the job monopolizing my time and me doing whatever work comes along when I’m on the clock, then that’s the job I took and I need to be available to them.

Yes. THAT is the expectation we are talking about. If a company doesn't expect that, then there is no issue.

in a lot of jobs the expectation is just to meet certain targets of work to be completed. If I meet those targets, the employer owes me the agreed upon wage.

Great. If that's what your employment agreement says there isn't a problem. Congrats on finding a white collar salaried job that involves no collaboration or expectations on availability.

If I pay the grocery store a dollar for an apple, am I entitled to as many apples as they can possibly deliver me? Obviously not.

That's akin to hourly pay. You work an hour you get $10. You buy an Apple you pay $1. Salary is like an all you can eat buffet. You pay $20 you eat 1 Apple, 2 Apples, 3 Apples, etc. But also there are rules; Can't take anything home, can't share your food with a non-paying person, you'll probably get cut off at some point if you eat too much or waste food etc.

If I pay a worker a dollar for a task, am I entitled to as many tasks as they can possibly deliver me?

I think this is the big misconception. You're describing contract work, not salaried employment.