Why are Norwegian classed as a LOCO?
This is because the term 'Low Cost [Airline]' refers to the cost element of the corporate balance sheet. It is NOT a reference to the fare structure.
Having a low cost base offers an airline the flexibility (but not the obligation) to sustain low fares. However, as businesses driven by the profit motive, such carriers will use revenue management strategies to maximise profits (from tariffs) just as any other carrier would do.
The frequently noted public belief that 'low cost carriers' by definition always offer low fares is probably the most widespread misconception in the airline industry. Tell your friends and family to check all fares offered on a desired route when booking flights; never assume that the 'LOCO' will be offering the cheapest deal to the customer.