The three off-piste skiers were originally convicted by a district court, but that decision was overturned on appeal to the cantonal court.
But the Federal Court in Lausanne allowed a subsequent appeal by the public prosecutor against the cantonal court decision.
In its ruling at the end of November, made public by Swiss media on Tuesday, the court said taking into account the trio's experience and the signs warning of avalanches, they should not have crossed the slope above the area where the avalanche hit. In doing so they violated their duty of care.
Given the marked risk of avalanche, their behaviour was likely to have caused it.
The ruling comes as skiers in Switzerland are being warned against skiing off-piste following days of heavy snowfall.
Nevertheless, on Tuesday morning a ski-tourer provoked an avalanche in Saas-Fee, setting off a 4m high wave of snow, reported Le Matin.
More than 50 rescue workers attended the scene but after three hours of work, no victims were discovered.