Maybe they trust it more.... For protecting their locations it would work nicely. Also it allows plausible deniability. Further if their actual comms are down, or bugged, it works as a backup system. Or even they could be using both which then if it's bugged they're only getting half of the story. Or even more they could be using real coms as a decoy and using this as the real comms. All I am saying is think outside the box. This isn't normal military ops. And most likely they're sending info to large groups of ex military or non military operatives, so they are using a public forum.