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...