Its a terrible thing to happen but how can anyone realistically police this?
It'd be really manpower-intensive to police them all, I'd say. You count on general human decency to get you something, and look what results...
I wouldn't have thought that old rusty steel would be worth much even as scrap metal?
I'm hardly an expert at it, but I think if the steel is thick enough, the outer layers of rust will slow down any further rusting deeper within the plates (kinda like a passivation treatment). So the thicker, high-grade steel of an old warship may still have a lot of good steel to... steal. And a modern steel furnace can process even old rusted steel fairly easily, so those ships are probably now high-rise apartments in China or Indonesia...