As far as I’m concerned, there really is no meaningful reason why weapons can’t be shared between characters. You can just implement some common sense restrictions to prevent the most obvious methods of abuse (level restrictions to prevent overpowered hand-me-downs, ban the transfer of anything class-restricted, etc).
Players should be able to actually extract value out of the rewards they are given. Otherwise those rewards are meaningless. Your mileage may vary, but it seems strange to me that one player should be rewarded greatly and another not rewarded at all just because (s)he happens to find things on the “wrong” character. It’s just another layer of RNG.
It’s also in Fatshark’s own interest, since the ability to transfer weapons creates an incentive to play more characters (and thus buy more content).