Why does he need to release the power? Other forsaken find themselves blocked...

 

Only when Shadar Haren decides to block them. You can channel at Shayol Ghoul, it's just very unwise.

also, Demandred is badass

But what can they possibly do, why is DO scared of his followers

He might find them holding the power to be rude. He probably wants them to enter meekly like servants, not full of the power and as strong as they can be.

Week 9 Question: We've read in the Forsaken's POVs that channeling in the Pit of Doom would have some...unpleasant...effects. Is this related to the nature of the opposition of the One Power to the True Power or is it the Dark One consciously acting against the channeler? If so, why should the Dark One care?

 

Robert Jordan Answers:It is a matter of the Dark One consciously acting, though interactions between the One Power and him, the source of the True Power, can be unpredictable. The Dark One is not pleasant. He is also highly distrustful. He…dislikes…things that happen outside his control or not at his order. Call him the ur-control freak. Combine these two facts, and anyone channeling in the Pit of Doom without permission can expect swift punishment on the assumption that failure to ask permission means you intend to do something he won't like. It isn't that he believes anyone can harm him, just that he is in charge, and your failure to ask permission, your presumed intention to do something he wouldn't like, means that your faithfulness quotient has just suffered a severe downturn. Myself, I'd sell you short in a skinny minute.

http://13depository.blogspot.com/2009/03/tor-questions-of-week.html