[dark_lord]

Is that because it is civil versus criminal, and you are using the word "illegal" in the sense of breaking a criminal law?

[mountaineer]

"Illegal" in the sense of violating a law enacted by the appropriate legislative body. I'm unaware that any state legislature, for example, has passed a law saying that people who enter into contracts must comply with them. That's why we refer to it as the "common" law.