I dunno I have never followed the other forums.
The BB site openly talks about steroid use and the weightloss forum discuss things like how to live off under 100 Kcals a day, 'instead of mashed potato, try papier mache' that kind of thing (maybe slightly exaggerated).
Can you imagine the chaos if those two groups started discussing diets

But I think you're right about standardising them to some extent, like having a single login for all sites and make switching between them easier with bigger links between the three