Here's how it works. Sega want to release their old games on a new system, in this case Wonder Boy. They own the IP of the game, but not the actual code. So if they want to release the game again, they need to either get M2 to recreate the game using completely original code (which would be a waste of time) or license the original code from Westone and then M2 can modify it to work on the newer systems. Thus, Westone would require a credit. Also why Sega should buy Westone.
Edit: An example of how this can go wrong is why you will never see Goldeneye on Nintendo's virtual console. They own the game, Rare own the original code and until last year Activision owned the 007 license. Very messy if you want to put the title out again.