Legality within Japan or HK is not the issue.
If their numbers people work out that they could sell 200,000 units of a Tank Megatron Masterpiece mould (domestically and globally through their customers, and later by Hasbro using the mould), compared to a maximum 10,000 units of a new Gun Megatron Masterpiece that has limited markets it can be easily imported/sold in (TakaraTomy's customers will only buy enough to sell domestically, and Hasbro won't be using the mould later).... which mould do you think they would spend their money creating?