If they tried to sell gas at less than market price, they would...

If they tried to sell gas at less than market price, they would have to be able to support it with some very sound arguments.
They may be able to find some argument, but they know the NWE shareholders will be screaming so they will be extremely careful with everything they do.

What NWE offers MIN is certainty of supply and posddible long-term contract, even if they pay market prices.

Lots of people here know a lot more than me about this.
Just my guess.