Why law and business faculty are paid the highest salaries?
I believe that there are (at least) two main reasons for this:


Law and business faculty generally have alternative job prospects that pay extremely well. That means that universities have to pay enough to compete with those alternatives in order to attract and keep their faculty.
They both generally teach in professional schools that are not part of the main university (at least in the US). Professional schools (especially top ones) usually have high tuition, high enrollment, and healthy donor bases (due to the aforementioned high potential income), and therefore they have very strong revenue streams. They also don't incur many of the costs that the rest of the university does (e.g. sports facilities, dorms, financial aid). Together, this means that they have significantly more resources to pay their faculty with.