Why are ethanol piston engines apparently cheaper to maintain than avgas piston engines?
Apparently a study was run for the FAA at Baylor University and it found that Ethanol caused less wear on the engines. You can trace the bibliography for quotes on the study. They concluded after testing for the O-235 engine 


  The relatively low wear and general engine cleanliness indicates that
  the engine can operate on ethanol fuel for longer time periods than on
  100LL avgas. An additional test is planned in which the engine will
  run according to the schedule met in this test, except for 300 hours
  rather than 150 hours, to justify an increase in recommended TBO for
  the engine operating on ethanol fuel.


The FAA also ran a wear test on E-85 that you can find here. Their summary is mainly about whether or not it works which it seems to, they don't touch very much on wear. Still worth taking a look at on the topic. 

It does seem that ethanol can increase engine life as has been seen in current testing.