Define sport.
Is it the ability to compete in a highly competitive game? If so, curling is a sport.
If by sport you mean, any game where you use balls, and strictly balls to play, then its not a sport, but then again, neither is running or rowing or boxing, etc.
Before you ask how something is a sport, why don't you ask why your games are.
It's like when straight people ask homofags, "Well don't you think you're gay because you haven't slept with a good woman yet?" and they don't realize that the question is ridiculous, because if turned around, they look like total weirdo..."Well don't you think you're straight because you haven't slept with a good homofag yet?"
Keep an open mnd dude.