Fair question. My immediate thought is that ATV's carry greater risk and that McDowell was more likely to be driving recklessly than Luck was to be snowboarding recklessly. McDowell is lucky to be alive, and Luck sprained his shoulder (the AC joint specifically). Looking it up, and with the info available from a quick scan, about 10 times as many people die per year riding ATV's than people do skiing and snowboarding combined. Overall injuries are about the same, but it seems that a larger number of people ski and snowboard. The last two facts were more difficult to verify with a quick scan.
I'd say that Luck's injury is more similar to a player hurting himself playing basketball in the offseason, and Mcdowell's is more similar to someone injuring himself while driving recklessly. I'm not arguing that players should become bubble boys away from the field, but almost killing yourself on an ATV points to remarkably poor judgment imo.