The owner of that site needs to shut it down. This kind of thing was inevitable and we warned about it from the start. Someone has calculated a rainbow table and the passphrase you chose is in it.
Which wallet software did you import the key into? Do we need to put a warning about this site into wallet apps? We need to find some way to kill this stupid and dangerous site asap.
I think we shouldn't make such of assertions without any evidence.
If someone calculated a rainbow table (and almost sure that have done more people) then it has nothing to do with the site owner.
It was the negligence of the user to use a simple password and the opportunism of a dishonest hacker which caused.
Is the Bank guilty if somebody take over your online account because you used 123456 as password ?
You shouldn't use something what you don't understand.