They're legal because they're laws. I think you might be asking how they're constitutional? They're constitutional because there's nothing in the constitution that prevents such laws.

Are they fair? I mean ... sometimes? These laws (supposedly) solved a problem where parents would bequeath their estate to their children prior the parents' death and, then, the children would leave their parents destitute. It's not clear how much of a problem this really was (think "19th century); it was probably just a popular image that politicians used as a talking point?

My suspicion is you'd need to use jury nullification to make the law "go away".