Maybe it has to do with the level of evangelism associated with a specific group. Beer drinkers as a group mostly couldn't care less whether you drink beer or wine. But political/social theory groups have an extremely high interest in everyone else's political affiliation and try to change it. Maybe it is because the ideals tie back to core beliefs or because the political beliefs of the masses do have big impacts on everyone's interests, but it is just one of those areas that people evangelize about, especially the extremists.

So maybe it's not that they joined the group for the explicit reason of pushing their political agenda, but are actually interested in druidry or masonry and happen to be extreme on the political dimension and just can't help but to push their political agenda regardless of what setting they are in.

It is just a theory, and maybe naive - but I work in an area shrouded in government secrecy and I so often see outsiders attribute random dynamics to malicious organized conspiracies that I tend to try to find alternate explanations before believing conspiracies.