Yeah you're 100% right that it's optional. It's usually only required to allow company data such as email, slack, file sharing etc on your personal device. If you're on-call it is VERY rare for an employee to win a fight on making the company provide a dedicated device for that purpose (which can inherently make it a condition of your job but that's an exception).

Most employees tend to not care about the why and are happy to just do it making "you" (the one bucking the trend) the oddball. The one not being the team player. It's not legally required, and you won't be fired for it, but its strongly socially encouraged and that makes it mandatory for anyone not willing to put up that fight.