The only reason I can think of they would do a thing like this is that they force people to exchange the blocked currency to another in stead of withdrawing, causing an artificial rise in stead of a natural fall.
Perhaps they are earning more with exchanges than transfers, or they have a large amount of the coin they blocked and are protecting the value.
Bottom line is that they are basically stealing. In my personal experience they blocked the BCN withdrawals from the moment the coin was at it's all-time peak a few weeks ago. The high could cause massive withdrawals and possibly deposits to competing exchanges. If they blocked the coin like they did people could only take advantage from the coins in their trading accounts by exchanging them on Poloniex and not another exchange, making them more money. Combine this with keeping the coin artificially high and you have quite a solid motive.