I have my doubts. It's not rocket science to introduce a mechanism which lets the advertisers choose what kind of videos are acceptable for their ads to appear in.

Don't like guns or beer? Tell Youtube not to displays the add on guns/beer related videos. Problem solved. Same goes for Reddit. Would advertisers care that there is gun related subreddit their ad never appear on?

I feel the decision is ideological and will hopefully hurt them in the long term.