It's true, investor who invest on presale/private sale that got a big bonus will sell some of their tokens to get a quick profit but remember bounty hunters also one of the reason why the price is fall after listed on exchange. No one can't control them though.
many of the bounty hunters did sell their tokens as soon as they were registered in exchange, it would indeed make a decline, but I don't think it would be too big. the effect occurs when the bounty makes prices go down and investors panic and join in selling their tokens.
Most bounty hunters can't wait to wait and they think that prices will drop even scam if waiting a long time, but I'm sure the bounty hunter is not a factor that causes drop prices, usually they only get 1% of the total sold.