Yong Zhang, a third-party Android application developer who sold a number of highly popular video game console emulators has been locked out of the Android Market. His applications were removed from the store and his account was terminated by Google without warning.

This is the second console emulator developer to get kicked out of the Android Market in recent history. ZodTTD, the developer behind a PlayStation emulator, was similarly sentenced to Android Market exile last month. These moves seem like a signal that Google is planning to purge the emulators from its mobile application store, but there may be other licensing issues in play here.

Although the unauthorized distribution of copied game ROMs constitutes copyright infringement, video game emulation is generally believed to be legal in principle. The courts have issued a number of rulings that put emulation on solid footing. An article in the UCLA journal of Law and Technology describes a case brought by Sony against a PlayStation emulation vendor Connectix. Sony lost the case because the court determined that Connectix emulator was not itself infringing and that Connectix's use of the PlayStation BIOS during development of the emulator was fair use.

As we recently experienced firsthand, however, big companies with large user-generated content ecosystems will often act on a bogus takedown request without even bothering to assess its validity. In such incidents, the accused has little recourse and generally faces an uphill battle to get their content reinstated.

The timing of the emulator takedowns is obviously going to raise some eyebrows—Sony Ericsson recently launched its Android-based Xperia Play handset, which has a slide-out panel with gaming controls. Sonly likely views Android's robust emulator support as a competitive threat to its own Xperia Play content sales.

It's not presently clear, however, who was the source of the complaint against Zhang's software. It's possible that other licensing issues are behind the takedown. Zhang's critics say that his SNESoid emulator was built using code that was misappropriated from snes9x, an existing cross-platform SNES emulator.

* It's technically not correct to refer to snes9x as "open source" because its licensing terms aren't compatible with the Open Source Definition's prohibition on restricting the field of endeavor for derivatives. Although this particular detail isn't really relevant to the issues at hand in the article, I felt compelled to point it out because it's an often misunderstood distinction.

The snes9x source code is published on the Internet, but large portions of it are licensed under terms that prohibit commercial use*. By selling SNESoid, Zhang might have fallen afoul of the snes9x license. It's worth noting that snes9x EX, a free Android port of the snes9x emulator, is still available in the Android Market and hasn't been kicked out by Google.

The official Android Market description for snes9x EX explicitly warns against buying SNESoid and other "unlicensed versions" from "scam developers who constantly remove and repost the apps under different names to rip off unsuspecting users."

The Android Police blog sought an explanation from Google regarding the takedown. The search giant confirmed that it Zhang's applications were removed due to violations of the Android Market policy, but the search giant declined to elaborate on the nature of the violation. Due to the lack of specific details, there is currently no way to know what instigated the takedown.

In the long run, the removal of the emulators from the Android Market will have little material impact on users. There are still plenty of other emulators in the Android Market and Zhang has moved his over to the SlideME market, where he will continue selling them.