Because it's a usurpation of my role as the administrator of my system. Because it necessitates a level of comfort with a company—whose business model is based on collecting and selling user data—rooting my system, and trusting that they're not going to abuse that privilege, or forget my best interest when acting in their own best interest. I think that's a good enough reason.
That is a good reason.
It's also possible to turn off the autoupdates.
It's debatable what the best default setting would be, and wether or not average users:
a) know how to disable this b) care to disable it
Like you say, it necessitates a level of comfort with a company. I suppose most people don't give this a lot of thought, and thus feel pretty comfortable with any company.
Wether this is "good" or "bad" is a separate discussion imo.
The only thing which you don't really know anymore is when or to what version it is currently updating. But why would you care anyway about that.