This last weekend MangaGamer has been experiencing issues with processing payments, and have explained the reason for it. Apparently for the last few months the site has had quite a few instances of fraudulent payment. Much of this activity has been from stolen credit cards and nearly all of the purchases were for games with Steam keys. Which would then be most likely re-sold.
With the limited resources the site has, MangaGamer decided on one measure to take to deal with the Steam key problem. MangaGamer will stop providing Steam keys, however any game you previously purchased that offered a Steam key will still be usable. Currently purchases still cannot be done on the site. MangaGamer’s titles are still available to purchase on Steam such as this one.
It’s unfortunate when things like this happen, but there isn’t too much you do about it. This kind of sucks since having a Steam key for one of their games was a nice little bonus. Although they’re not optimistic about it, maybe one day they’ll start offering the keys again.