People get paid for all sorts of reasons. If you are being paid specifically to put work in front of your hobbies, then that is your job! Think database uptime employees who are paid to get up in the middle of the night to fix a bug, etc. Heck, I even paid a friend to eat a shoe once; you can pay people to do all sorts of things. Paying someone to put work in front of hobbies is hardly novel or extreme.
Of course, this should be clear in the job description!!
...then it is most probably illegal in quite a lot of countries. Labor codes exist for a good reason. Most people are a) employees b) not willing to put life ahead of work and c) not willing to be forced to by competition from desperate/workaholic co-workers.
Database uptime employees work shifts, so even they should not be forced to put life before work.
Do you think it's reasonable to expect a CEO to put work in front of hobbies?
Nevertheless, how numerous are CEOs compared to other employees?
Legally, as an exempt employee, at least in California, my employer is in the clear. Hell, I believe they do it to hourly workers, too. I have worked in the computer industry my whole life, but I am told that it is not uncommon for minimum wage jobs to insist you come in at unexpected hours to cover unexpected events.
You're buying his time and experience for agreed upon hours. You don't have the ability to make him put your work ahead of his anymore than you have the ability to make him put your work ahead of his family, personal life, education or retirement savings.
In discussions like this I tend to fall on sympathizing with employers because until you've taken the risk, hired and managed people you don't fully appreciate just how hard it is or how difficult it is to keep people happy.
What you're describing is closer to buying his life - which would be an understandable arrangement from a business partner but not an everyday employee.