OSCON The Mozilla Public License (MPL) is the latest casualty of Google's decision to remove open-source licenses from its popular code hosting service.

The search giant has said Google Code is no longer accepting projects licensed under MPL, although existing MPL-licensed code is allowed to stay.

The move comes two years after Google Code launched, when MPL was one of just seven licenses Google allowed developers to use. Others included Apache, BSD and the Free Software Foundation's GPL and LGPL.

Google's MPL ban follows the block on FSF's Affero GPL. That decision's seen a number of projects abandon Google Code for rival hosts.

As with Affero, the reasons for Google's decision are not entirely clear.

On the one hand, it sounds like Google is using its critical mass to force an industry reduction in the number of open-source licenses. Open source programs manager Chris DiBona said - as with Affero - the move is designed to stop license proliferation inside Google Code. A decent objective.

He told the O'Reilly Open Source Convention (OSCON) developers are welcome to host MPL projects elsewhere, just not on Google Code. He noted you could still build plug-ins for Mozilla's Firefox browser without licensing under MPL.

Here's where the proliferation argument stumbles, though. DiBona said MPL is not widely used to justify a place on Google Code - also the reason given for blocking Affero.

OK, so proliferation as a concept is bad.

Proliferation is not bad, though, when a license grows in popularity. Two things are needed to pass muster on Google Code: to be used in "thousands" not "hundreds" of projects, plus a dose of "gut measure" from DiBona and his team. "It's so arbitrary," DiBona told The Reg.

It’s Kafkaesque in its simplicity.

It really, though, sounds like Google is concerned about the threat some licenses might pose in terms of code authors and hosts (Google) getting prosecuted by holders of intellectual property (IP) and by the degree of ownership over its own code Google might have to surrender.

Recalling Microsoft saber rattling (here and here) over IP in open source, DiBona told OSCON: "Most open source licenses have not a thing to say explicitly about patents or trade marks."

He added: "That's the reason we use the Apache license as the default license," for Google's projects.

So, no surprise: Apache is considered "safe" for developers, and safe for Google's own business.

This could certainly explain Google's aversion to Affero, which says companies like Google running services are distributing code and must allow consumers of those services to modify and pass on that code. That could threaten the Google's secret algorithmic sauce.

MSL, though, seems relatively harmless. And, how can Google dump MPL when it still seems to accept GPL, a license Microsoft takes strong exception to.

As DiBona told us of the decision to drop MPL: "We [Google] will be unpopular for a while."®