Q: if i purchased my device with my money is it legal for someone else to put on the parental controls?

Yes, if you are a minor.

That is an interesting question.  If you are at the "age of majority", then why would you allow someone else to put restrictions on your device..  If it is a condition of your employer to use the device on their network services, then you allow it to use the device for their system.  If you are under the age of majority, and your parents are requiring it, sounds like the parents care.

If you were a parent, then you would not ask this question.

Their house?  Their Rules.  That's the way it is in my home.  If my son is paying his own phone bill, then it's his device.  If he is using my phone line (The bill is in my name), then I could require restrictions.