If it is worth to mention:
Ether delta(already hacked):

Fork delta:

Idex:

Stellarterm:

And so on...they all ask for private keys.
If I am not wrong, metamask should resolve this problem.
The thing is, I am not really sure about this one and I have more than 1 opinion. I just don't have clear answer and I understand your doubts, especially when someone ask for private keys.