I can't speak for /r/badphilosophy. It's exactly the kind of airless, cliquey, snarky, injoke-ridden, self-obsessed shitpile into which every online community of academic philosophers degenerates. I see this is another instance of a pattern Paul Graham noticed.
I think the important thing about the real world is not that it's populated by adults, but that it's very large, and the things you do have real effects. That's what school, prison, and ladies-who-lunch [and academic philosophy] all lack. The inhabitants of all those worlds are trapped in little bubbles where nothing they do can have more than a local effect. Naturally these societies degenerate into savagery. They have no function for their form to follow.
Having said that, I think there are genuine reasons why one might be dismissive, even contemptuous, towards Sam Harris.
He doesn't have any expertise on the subjects he writes about. In fact, he doesn't really have expertise in anything. As a result, he never shows more than a surface-level understanding of any given topic.
He's not an original thinker. He's an excellent writer, able to be clear, logical and humorous at the same time, but nothing he says goes significantly beyond the observations and insights of other people.
On the specific subject of ethics, he's annoying because he never (afaik) acknowledges (i) that what his approach consists of is adopting "morality = maximizing human flourishing" as an axiom (ii) that there's no purely logical, purely scientific way of justifying this. I think much of the friction between Sam and philosophers would disappear if he simply mouthed those words and carried on, exactly as before. He fails to do this because he doesn't understand ethics at more than a superficial level. (He views it as a boring waste of time to dig deeper. This will make people dismissive and contemptuous even he is correct.)
This is beside the point, but personally I don't dismiss Sam Harris, because I think I he has an admirable lack of intellectual dishonesty. His role is to act as "the little boy pointing at the naked emperor".