Hurtigruten is partly subsidized by the Norwegian government, as it is considered a public necessity, offering scheduled travel between points where public transportation might be scarce.
Thus, a number of cabins is supposed to be reserved for point to point travellers.
There even is a sort of "cabin guarantee" for walk-up travellers (1-5 PAX, four nights travel or less): one is guaranteed a cabin if one turns up on the quay at departure, unless 120 berths already are sold to point-to-point travellers. How this can be enforced, I don't know (especially how a customer could know that on a full ship how many passengers are point-to-point and how many are cruise passengers).
Hurtigruten has now opened booking for Alesund to Trondheim in June 2018. I checked two days ago and there was nothing, so this is since then. Unfortunately, the day I want to travel shows no availability. I sent a note to the email given in thisi thread but wondered what wise and calming words you have for me.