That may be true in the US... but you can still find many recommendations preventing the use of APUs...
Noise and air quality seem to be the main concerns.
Vanvouver internation airport :
http://www.wyle.com/ServicesSolutions/science/EMMA/AcousticandVibrationConsulting/Resources/DocumentLibrary/Documents/NoiseManagementProgramPractices4-10%20Final.pdf
Manchester airport :
http://www.manchesterairport.co.uk/manweb.nsf/alldocs/8100FB8EF658808C80257364002D85FA/$File/NoisePlan.pdf
Search the word "APU" in these documents.
And by searching "airport apu noise" in google, you'll find many more.
This list is interesting : http://www.boeing.com/boeing/commercial/noise/list.page
You'll see some US airports listed with APU operating restrictions : LAX, SFO, BOS, ...