This weekend, Apple banned top-grossing chart-topper Monster Strike from the App Store for several hours. What happened?

Monster Strike is, as of this writing, back at number two in Japan's top-grossing chart, as is typical for the hit game, which pulls in $4.2 million a day. Its publisher, Mixi, appears to have run afoul of the fact that it distributes items to users outside of Apple's ecosystem via special codes which users can input into the game. These items are thus outside of Apple's ability to collect profits on them.

That analysis comes from expert Japanese mobile consultant Serkan Toto, who wrote about the event on his blog.

He pegs the losses for Mixi during the downtime at $600,000 -- meaning Apple lost a hefty chunk of its own fees, which it must have considered worth it to make the point to all Japanese mobile studios: if you break its rules, you will be punished.

This means, in other words, that distributing premium items without using Apple's in-app purchasing system looks to be a way to get on the mobile giant's bad side. Another Japanese mobile studio, Colopl -- whose White Cat Project is another stalwart top-three grossing game -- dropped the practice earlier this year, Toto reports. One can assume Apple warned both publishers, but only Colopl caved before punitive action was taken.

There's more on the ins and outs of the issue, and just why Monster Strike's system probably ran afoul of Apple's rules, at Toto's blog.

If you're scratching your head about just what Monster Strike is, read our interview with its lead developer, former Capcom R&D head and Street Fighter II producer Yoshiki Okamoto.