It is just a manner of fact that companies reserve their right to fire people to work for them with little to no excuse, and that is something we must live with, at least until there are changes of laws.
Yup, that's the life of being employed. We can be removed anytime the management wishes to. With politics or not, with something good or bad that an employee did, always think that they have the rights to do whatever they want to retain or remove an employee. But on this case, they all have the right to impose that rule because that's not right at all for an employee to do something inside working hours that's not in line with company's business.
One of the risks when you are really just that depending into a single source of income or having that day job is that you can possibly be kicked out or laid off because of those common reasons. As much as possible you will really be needing to have that multiple source as much as you could even if its hard but at least you are doing your best on trying to make your living way more better. Just be sure that you will really be obeying companies rules and policies and as much as possible on trying out to avoid on things on which you do seem that it will really be able to affect your work and possible that being removed due to some reasons.