For all of my 60 plus years as a skier, one of the few primary rules has always been that the downhill skier(and now also boarder) has the "right of way." Another is "always look uphill before proceeding." Both are continually being violated!!
Even though the snowboarding stance does position the boarder facing down the mountain more than a skier, it does not negate his responsibility to look uphill before moving into/across the "flow of traffic."
I see and experience this with boarders(and skiers) all the time, and as suggested above, I will deal with it in my speed and positioning, accordingly. However, this does not absolve the "violator" of their responsibilities.(We should always try to be aware of any merging runs, trails, etc. ; I also see many participants jumping into regular runs without proper spotters.)
I think that a major part of our problem lies in the fact that, the majority of new participants, in the sports of "downhill sliding," (particularly snowboarding) do not take formal lessons and miss out on any instruction regarding on hill etiquette. That being said, I don't even know how much these basic rules are still taught in current ski/snowboard lesson formats.
Maybe they need some "basic rules of the hill"(~10,) clearly read and signed off on, before they are able to procure a pass(season or daily.) This would be a major hassle, but unfortunately, our world forces us into many of these things. And it might not make much difference but every bit helps. (Lift passes have liability clauses on them.)
In the OP's posting and his add'l comments, it sounds as if he was mutually responsible for the collision.