Plenty of jobs that you can’t sit at.

Yes, but if OP is using the computer half the day, you should be allowed to sit while using it, especially if you have to hunch over. If its a standing desk then OP has to wait until breaks/lunch. OSHA apparently does keep on top of workplaces going against ergonomic safety, and that includes having to use a computer for long periods of time hunched over with no relief. (Includes more and additional things per state, but this is the issue at hand.) https://www.shrm.org/resourcesandtools/tools-and-samples/hr-qa/pages/ergonomics-do-employers-have-to-provide-ergonomically-correct-furniture-under-osha-requirements.aspx If OP is in helpdesk IT, I feel bad. Yeah you're helping people half the day but you're also doing stuff on the computer remotely. Not sure whats going on though.

Not exactly a double standard. Boss wants you and hired you to be there to work. Your boss owns/runs the company and delegates his work to you. He can do whatever the fuck he wants! Doesn't mean its going to feel or look nice on him. The boss is an ass, especially if OP is just using it for entertainment during his lunch break.

That is the definition of a double standard right there.

It'd be a double standard if it we're two workers with different allowances, the guy is complaining his boss/owner doesn't have to work and his literal rules for employees states they're not allowed to do what OP said. It's OPs job, not a double standard.