Again, what kind of brightline are you asking for here? One where you cannot control how your own server responds to requests? One where you cannot determine the content of your own website?
I don't know what the bright line is, no one in the comments has been able to come up with one that defines ads in browser as bad (especially because they're not the only browser to have ads -- *waves to Firefox and Brave*) but wouldn't affect other forms of advertising we already find acceptable that isn't "I just don't like it." And that's fair, I don't like it either but we've kinda painted ourselves in a corner with the whole, "software is the property and platform of the who make it" thing.
Transmitting me a piece of code I run on my machine or voluntarily install as part of the OS isn't really any different than what we do with browsers anyway. After reading everyone's comments I'm leaning toward, "these ads in particular make tech people uncomfortable because they're well targeted and it breaks the mental model they've mistakenly held that the browser is actually theirs and not serving the interests of corporate daddy."
Big Stallman was right energy.
And if I accept your ability to place advertisements on your website, surely you're not saying you feel motivated to argue that AT&T should be able to inject content on competitor websites? And Samsung too?
What kind of brightline are you trying to draw here?
As a user, I just don't want browsers to inject content when I'm viewing websites. I also want owners of websites to be able to say what they want. What are you trying to argue for?