Because it the same what developers do. They convert all funds to US dollars and then don't care about ETH and BTC price. So bounty hunters do, because token price drop 90% and this is normal.
I have not read any proof that developers are often doing this and your reason might not be correct. Most blockexplorers don't show ICO funds being moved.
in any case, I think that the tokens are falling due to the drain of coins by investors, and not by the Bounty members of the company. The average total pool of Bounty is a company of 2 or 3% and this number of tokens can not influence pricing, when entering the market.