This DLC is actually not necessarily just money grabbing. While I am not sure if SE does the voicework themselves, I know that voice acting companies often charge in accordance with how big the potential audience is for whatever it is they're making.

It is actually entirely possible that a VA company would charge a lot more money to make japanese voices that would be available for the entire world rather than just japan. If they were included in the game, they might have to pay for every english-speaking customer who never are going to use the japanese voices. However, by making it DLC, they can easily see how many people bought the japanese voices outside of japan, and only have to pay the VA company for those who did. As an added bonus to "early adopters", they made it free for the customers the first few weeks, even if SE might have had to pay royalties for these downloads even if they were free for us.

In short, giving every customer access to every voice language can actually cost more money for the developer/publisher. It's not necessary "free" to do just because the voices have been recorded anyway.