I think some have issue with them collecting monies on behalf of an organisation who has made no indication they are interested in participating. If my site gets 1000 BAT in payments which sit there because I decided I don't want to take part, what happens if I say to Brave I never intend to take part? Do they give those BAT back? Site visitors may have had the impression they were paying for my content while Brave is pocketing it.
Personally I reserve judgement, but I see why some look at it this way.