Why FAFSA asks for parental fin'l info for law & med school students only, not other grad schools?

I just filled out a renewal 2017-18 FAFSA for my daughter, who finished Bachelors in 3yrs and is going to law school right after. FAFSA instructions say that since she's considered independent (by virtue of being a grad student), she does not have to fill out parental income info, but they recommend that it'd be filled out if the student is going to a law school or a med school. 1. Why law & med school are singled out by FAFSA instructions? 2. In which cases are we better off providing versus withholding parental info? Thank you for any help or advice specific to these questions, MK