Unlikely anything flying to Europe (short of some tiny airline flying cargo in a DC-6) will be using AVGAS. So they're paying closer to $3.80-$4.50 per gallon for jet fuel.

As to why: aviation fuel is a specialty product, and quality control has to be a lot higher than road fuel. Get some water in gasoline, and some cars stall out and need to have the fuel system drained. Water in aviation fuel might mean landing in an inconvenient (read, wet or rocky) spot. Plus it has to maintain its characteristics from 50 below to 100 above zero, maybe on the same flight.

Even with lower taxes, this adds up.