Indeed not. But "it's not stealing if I'm stealing from thieves" is a poorly thought-out moral argument at best. You're still helping to prevent the item from being returned to the person it was stolen from.
And again, the owner might not be a human, but a country, business, museum, laboratory, school or...well, you can continue to fill in the blanks. Suffice it to say, that the passage of a mere 50 years may not be enough to extinguish the possibility of the existence of rightful owners.
In which case, they won't claim it, and you'll get the items back, free clear and legal. And if they do claim them, then it would have been wrong for you to keep them. It's self-regulating.