Why would you care whether or not it's legal? The fact that it happens many times daily is enough for me.
Interesting. I have to say that I agree with virtually every post here, including my own. Yes, markets do get manipulated from time to time. And no, it is not illegal (usually), and no it is not bad for traders if you know what you're doing.
Especially the overnight session WAS once the beloved playing field of Steve Cohen, CEO of SAC Capital. Don´t know whether Stevie is still "active" at night...