It's not just Norwegian, but all carriers do this although it's not quite as transparent as Norwegian since they don't offer one-way fares like Norwegian does. For example, the cheapest fare on Delta Airlines AMS-SLC in May 2016 is EUR949 which converts to about USD $1,048. But if you fly the same route in reverse originating in the U.S. SLC-AMS, the cheapest fare is $1,462.

As another example, the cheapest fare ARN-LAX in May 2016 is $759, but the cheapest fare LAX-ARM in May 2016 is $943.

In playing around with fares I'm interested in on Norwegian, I definitely see that NOK fares are consistently lower than other European currencies by 6% or so and lower than USD fares by a whopping 24%. I'll probably pay the 6% penalty and book under all other countries English just so I get communication in English.