Hate to start plugging holes here, but unless snowbarders only stand on one foot, equal weight is spread on both legs. Yes they may lean back in pow putting more weight on their back knee, but skiers also shift weight from leg to leg while turning, which shifts the stress from knee to knee. Snowboarding is actually easier on your knees and ankles than skiing, but wrist and head injures are more common in snowboarding.
You cannot debate what's "easier" because there are equal pros and cons to both. For example while skiing has two skis to worry about, that added challenge also allows the skiier to widen and adjust their stance to adapt to increased challenges on their balance, while a snowboarders stance is fixed.
So asides from the "fuck them" concept which is just not a valid point, talk to some doctors or do some research on the injures prone to skiing and snowboarding and base your argument on that.