How is it more acceptable because it's a combination of two things? Lots of objectionable outcomes are a combination of multiple factors!
I wouldn't say it's reasonable that browsers could never suggest they be made default, and I don't think it's reasonable that you can't have some OS function that wants to show e.g. a help section use the OS embedded browser.
I accept people must work at height. I accept that people occasionally fall over by accident. We have guard rails and harnesses so that I don't have to accept people falling to their death every time they trip at height.
For example it wouldn't be ridiculous to think that if the browser is tightly coupled with the OS (to the point that it doesn't change when you set the default browser) that you can have the embedded browser opened with a no nag/no ad flag set.