The system owner. Usually that is the same entity that owns the secure boot keys, which can be the person that bought a device or another person if the buyer decides to delegate that responsibility (whether knowingly or unknowingly).

In my case I am talking about myself. I prefer to actually know what is running on my systems and ensure that they are as I expect them to be and not that they may have been modified unbeknownst to me.

This is only the case if the person sitting in front of it does not own the keys.