It seems that these issues are related; hard work is a virtue that God rewards, not the direct cause of good results.

Actually, the Gita says that you "have the right to work, but not the right to the fruits of your work", which I always interpreted as meaning that one should take pleasure in the work itself, because excessive outcome-dependence is bad for your peace of mind.

But it would also make sense if a common interpretation of the phrase is that work is a virtue admired by the gods, or even that everyone has a societal duty to uphold through work, even if all the benefits of that work go to others. (After all, in the verses all around that one bit of useful advice, there's an awful lot of Lord Krishna talking about how various evils lead eventually to such horrors as the mixing of castes!)