The more I think about it, I am not so sure that United doesn't participate on FlyerTalk. But I believe they are using intermediaries instead of the official "UA Insider" account. It seems to be in the same camp as using "bloggers" to put forth UA in a positive light. Eliminates the pesky problem of actually having a dialogue and answering questions.
And United absolutely is monitoring FlyerTalk. Former UA Insider Scott openly stated it was a very valuable information source for their Customer Insights.