I don't see why it would be illegal. Technically, this website is designed to test your own network agaisnt security flaws. Which would be the same as hiring ethical hacker to test your security.
The only real issue is that they only accept bitcoin, which mean that it is (almost) impossible to know who is behind the attack. I think that is the issue. The service they offer is totally legit, but the way they sell it makes it way too hard to know who is commanding the attack. Therefore, I think that the company should be responsible for the use of their software.
Kind of like if a gun seller refuse to give the police a list of his customer after a shooting...
@pudubat - Exactly as @hotaru251 said. They're NOT in the ethical hacker business, because they'll attack anyone you ask them to. That they also only accept bitcoin , likely to hide who's hiring them, simply makes it more obvious.
Where do you get the idea they're a security-testing company? What at all gave you any indication that their policy is "we'll only hack websites that you own"?
There isn't any such indication. That is obvious from this article.
A doctor can legally remove a patient's appendix, with the approval of the patient for the medical procedure. That doesn't mean that the doctor can legally start up his own service where you send money and he just goes perform appendectomies from people you request.