The documentation for ABCMeta.register() says that it makes the other class a "virtual subclass".  That would make the ABC a "virtual base class".

So whether the current behaviour is correct depends on whether you consider a "virtual base" to count as a base.  From the reasoning behind the introduction of ABCs, it certainly sounds like it should count.

Also, this is a feature that works correctly in Pyton 2.7, so could trip people up who are trying to move to Python 3.