I don't think there is anything illegal about it. It has been quite a few years since I've had a safe deposit box at a bank but vaguely remembering in the agreement you were not supposed to keep cash or precious metals. I'm not sure if this is a bank liability thing or perhaps a security thing or what. Maybe if the bank is known for having customers keeping lots of cash or PM's they feel it puts them at a higher security risk? Maybe there is a banker on here that can answer. We kept silver coins my wife's great uncle's left her for years in the box. Nobody ever ask what was in there or made a fuss.

I got a feeling it is a bank thing, not a legal issue, but don't know that for a fact.

John