IB wants another source of profits. Under the cover of risk management, they are increasing their profits through extra fees.
They will push the envelope of extra fees until traders start fleeing. From the look of the posts, it seems like no one will leave IB over exposure fees. People are annoyed, but not enough to leave. They have done their job for IB shareholders.
I think traders are more calculated so they phase out and look for a new broker rather then just emotionally closing positions. The same way, good tarders manage their positions so that the 'unlikely' scenario of the exposure fee will never happen.
My self, I have not posted any trades on IB since this fee and have found a good alternative with similar commissions and margins but no exposure fee for the instruments I trade.