interesting... here in Europe it always states how many hours employee needs to work daily, how many days per week and often times includes clauses for how much overtime hours and overtime pay is to be expected
Yeah if you interview in the US for a tech job typically they will tell you that they have "occasional" overtime. I've seen occasional range from a couple times a week to a couple times a year, always unpaid. You are being paid for filling a role, not a specific set of hours. People talk about how "my boss doesn't care so long as I get the work done" as a benefit but it's actually how the salary arrangement is supposed to work legally speaking. Many roles are misclassified as salaried-exempt (You are paid a salary, and exempt from the overtime laws) in order to step around overtime, there are specific requirements around it like you having enough ability to influence your work in a managerial or creative capacity.
Company time only exists when the company is paying for time, salaried employees are filling a company role not being paid for any specific period of time. If the company gets too specific about what time belongs to them they risk losing the exempt part of salaried-exempt and would owe back pay for any time over 40 hours that you may have worked.
how does one measure how much work can employer extract from employee if position is 'filling a role' as opposed to having an agreement for amount of hours per day?
The minimum wage always applies, so you'd have to pay at least [168 * $7.25] $1218 per week if you wanted someone to work 24/7 365. Had the minimum wage kept up with inflation that would've been somewhat of a limiting factor. The primary limiting factor is "at will employment", that is a doctrine where you do not owe your employer any notice when you wish to leave your job. So if they tell you "Work 100 hours a week" you can tell them "no" and leave without legal consequences. In practice there is a informal understanding that you will provide a employer with two weeks notice when you leave, and they will often give you a poor professional reference if you do not oblige them.
Another wrinkle is most workers aren't really aware of the legalities of salaried work, you'll see people making statements like the OP here about "company time" for example. Originally the arrangement was designed for people like CEOs and executives of companies where it wouldn't make sense to track hours as they had a stake in the company, but it has been expanded mostly to side step things like mandatory overtime pay.
Company time only exists when the company is paying for time, salaried employees are filling a company role not being paid for any specific period of time. If the company gets too specific about what time belongs to them they risk losing the exempt part of salaried-exempt and would owe back pay for any time over 40 hours that you may have worked.