It is because in doing so you bypass the normal "rules". Effectively you put your employees out of the game when it comes to negotiations.
This will become really really ugly.. Simply put Mr. Bjørn Kjos big dream is to be able to compete or beat the lowest production cost out there, meaning for shorthaul it is not enough to beat Easyjet on costs, no he want to beat Ryanair .. It is like an obsession.. Base meeting, after base meeting, up comes the spreadsheet comparison to other carriers.. and one name comes up.. Ryanair, Ryanair, Ryanair.
In his quest to drive costs downward Norwegian has been reorganised, so the company you purchase your ticket with is a "virtual airline" with owned production companies underneath. The Kjos logic now is that if one "unit" is on strike, he can just ask another unit to fly his aircraft.. For negotiations production units can be put up against each other to drive down T & C's.
In short the non-scandinavian based crew will be used to break down T & C's of the original flight crew that helped build this airline.
The only solution is for all flightcrew to stand united. This is probably also the only chance of a turn-around of the contractor policies introduced the last couple of years.
None of the scandi pilots want Norwegian to become a new Ryanair! I am not sure Kjos is aware how pissed off people are.. He probably will never show up again at base meetings, in Scandinavia ;-)