Workplaces not bugging employees after hours is a great rule of thumb, but blanket rules don't leave room for people to use their judgement to resolve (hopefully very rare) legitimate emergencies and problems.
It's probably tied to their actual work agreement/contract.
If one of your explicit work requirements is to be on-call, then it's likely fine. That way employees can also argue that they should get paid for that time as well (and usually, on-call agreements also have compensation rules as well).
If you can refuse the on-call agreement and still keep your job, then it is okay. Otherwise, it is an option for employers to take away their employees' free time as they wish.
The cost of my time as an employee is not linear. Just because I am selling 8 hours of my time for X money, doesn't meant I would be happy to sell 16 hours for 2X money. And if I am taking a week of a vacation, it means I want to relax and walk somewhere in nature and be left alone. If you want someone available 365 days a year, hire two people.
