I was taught in the Boy Scouts to "be prepared" and always carry: a pen, fire, and a knife. I wouldn't stop carrying a pocket knife even if they were illegal (I shamelessly carried one while in the UK, because why? because f**k them, that's why).
Most laws are petty and spiteful, this one included.
As a violent felon (he walked into the Federal Reserve with a sawed off shotgun previously) he was also ineligible to own a firearm. So another extra special law to prevent truly crazy felons to commit super illegal acts of murder probably wouldn't have stopped him either.
Stricter hospitalization laws for the mentally insane might have, but those are discriminatory against the murderously insane. So they were abandoned in the 70s.