Linux distros won't host the code for the commercial bits. It doesn't matter if you can hide it, it's the fact that it's there at all

Yet they have no issue with Mozilla?

Mozilla does not have commercial bits. They do receive money from Google to be the default search engine, and the binaries they build report telemetry, but the versions found in Linux repos often either patch out the telemetry or disable it.

Most distros have a way of installing proprietary software via enabling additional repos after install.

And you can do that if you want with Brave

For technical users who are in the know, yes. I would not recommend Brave to less-technical friends and family knowing that they would surely be duped by some dark patterns in Brave's UI/UX.

Even Firefox, which is the best we have currently, surprises us a few times a year with questionable decisions. Still, it's what I recommend to people.