Brave is perhaps the most ethically challenged browser out there. Hopefully they have stopped doing this, but they were injecting their own ads instead of what the publisher put on their website.
The reader can guess how Brave expects to make money with a free browser that is handing out BAT :-)
Personally I reserve judgement, but I see why some look at it this way.
Of course sites can block requests based on Brave's user-agent string if their business depends deeply on ad revenue and they consider users with ad blockers to be abusing their service. That's their prerogative if this really irks them and it's worth losing the users. On the flip side if this becomes popular enough then site owners see real money on the table and they'll opt in to picking it up. That seems like an easy fix for them. If I was a site contemplating either blocking content to users with ad blockers or allowing cooperative users to opt into a more private client-side ad experience which still gives me the opportunity to collect revenue for their traffic, I'm pretty sure I'd choose the cooperative approach.