I personally believe it is all about which Boards managers think may help promote the service more and get more visibility for it. I do not always agree with their choice as some times I consider at least one of the Boards they disconsider to be actually helpful for the service promotion but it is what it is and I am guessing they do know best.
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I consider these two to be the likely reasons: politics is dirty, and some participants may use this section to attack persons whom they hate in politics,
This happens on most of the other Boards too however. It is not a strictly a 'Politics & Society Board' behavior. Even I some times express political views in the other Boards and 'attack' who I hate in politics. In my case, pretty much everyone!
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My personal opinion is that people should be allowed to behave naturally and not encouraged to participate in topics where they cannot contribute to the discussion in a proper way. A campaign that focuses most of its posts in only one part of the forum does not have nearly as good visibility as those campaigns that give more freedom to their users.
Speaking from personal experience with this kind of situation, I agree. I am more than convinced that it is a very clear difference between the quality of my posts when I have the freedom to express my self and to write where I want versus when I have to post in a particular part of the Forum in order to qualify for earnings.
At some point it becomes too much anyway and posting feels forced. And then what is the point. Bitcoin Talk should be a fun place to spend time around.