Higher level education is largely voluntary, and it's up to
the person taking it up to decide what they gain from it.
If they are only in it for a diploma, they'd get there one
way or another.
Then again, I have a hard time envisioning what would someone cheat about in a legal exam, other than not memorizing the things, but that's to me just a signal that the content is badly presented (instead, put students in a pretend courtroom in a case that covers the study material, and they'll have to learn it, and learn to apply it).
But even if we accept it as so, it is not an argument to be so vigilant in catching cheaters majoring in other subjects.