I've no idea if SBF told his parents what dodgy stuff was going on or not, but if he didn't then is it unreasonable for them to think "our son is rich from being a successful entrepreneur, of course we'll accept gifts from him"?
I feel they should only be in legal trouble if it's proven they knew laws were being broken. I don't run a criminal enterprise, but if I did then I'd sure as hell give my parents a version of the truth that excluded criminality...