I do agree this happens because it's a relatively small market.
However, these exchanges don't work as brokers, but rather as sportsbooks (that's why GDAX and COINBASE suffered so much while others did not), and there's a really long way to go before markets can no longer be manipulated.
Imagine billion dollar hedge funds entering the market, leveraging their positions.
How much money has to be poured into cryptos to prevent them from playing with us at their will?
Are we going to reach that point some day?
Remember that stock exchange and forex work under certain rules, with severe sanctions.