ASCII is Python's default encoding, unless the environment specifies otherwise. It's probably not a good idea for PA to impose a different default to the default default (!) because that has the potential to break code which wasn't written with i18n in mind.
Code which was written with i18n in mind shouldn't depend on the default encoding, it should always explicitly encode all external input to
unicode objects by making a concious choice about the encoding, and such code will run fine regardless of the default encoding.
Hence, defaulting to ASCII is the best option for services that want to remain compatible. Also see this SO question.
Of course, Python 3 makes all this better, so once everyone's adopted that then life will be good. At the present rate of adoption that might even happen before my 6-month-old daughter graduates from university (but I wouldn't bet on it).