I found out from some local members of the Amish community that the reason they reject grid power is that it would put them in ongoing debt - which is very much against their religious beliefs. This is also partially why they reject the use of automobiles (some communities are more permissive and allow the hiring of vehicles to drive them long distances) - purchasing one can A) put you into debt which is against religious beliefs and B) goes against their beliefs of being good stewards of the earth (ongoing greenhouse gas emissions and other pollution).
They don't reject use of things like some modern power tools, etc for use in their outside work for the 'English', especially when it comes to roofing contracts, building RVs, sheds, furniture, etc.
What's interesting to note - the debate is still ongoing in Amish communities about solar power panels and if they will be formally allowed - on the one hand it will give them electricity and whatnot in a 'clean' manner in compliance with their beliefs, on the other, there is objection to some of the materials used in those devices and how those materials are resourced.