I think pilots in the US forget that all of the US is controlled airspace by ICAO definition - class E everywhere from 700/1200ft AGL. So any airway will be in CAS and require a clearance to fly IFR. Whereas the U.K. Is mostly class G.
Not quite all true. "Normally" Class E starts at 14000 feet (in the same way that "conventional" aeroplanes are taildraggers). It's true that in populated places it starts at 1200, sometimes down to 700 and even sometimes down to the ground. But in unpopulated areas it's all over the place. Take a look e.g. at the area around Las Vegas, where there are incomprehensible little corridors at odd altitudes.
In any case in the US, even to fly IFR in Class G (which most US pilots have never heard of and don't believe is possible) you still need to be on an IFR flight plan.