The Tuskens have apparently had a long history of adversity against external settlers on Tatooine.
According to Wookieepedia the Tuskens were first disturbed by a group of settlers around 5000 BBY. These initial hostilities continued for centuries, derailing continued efforts to colonize and develop the planet.
After Tatooine was rediscovered by the Galactic Republic in around 5,000 BBY, early Human settlers were believed to have disrupted the water-supply of a settled cave-dwelling society known as Ghorfa culture, precipitating the transformation of the natives into the nomadic Sand People. To survive, they were forced to steal and adapt the technology of the colonists, forging the distinctive desert survival gear by which they would subsequently become so well-known. By around 4,000 BBY, they were also engaged in endemic low-level warfare with the settlers, raids which were among the factors that forced Czerka Corporation to abandon their attempts to operate Tatooine as a mining world...
Fast forward a couple millennia to around 550 BBY. While outsiders continue to attempt the develop the planet, an outsider gains the trust of the Tuskens and slaughters the local colonists. Then he kills the Tuskens. This likely precipitated a whole new level of hate in the Tuskens.
Alkhara ... [was] studying the Sand People, and seemingly gaining their trust. Eventually, however, he turned against the colonists and occupied the desert fortress that was used in earlier centuries...afterwards [he] turned on his Sand Person confederates, and destroyed their camp. This, it is claimed, was the source of a subsequent blood feud between the natives and the outlanders.
A couple of centuries later in a new colonization push the Tuskens are still holding their grudge. They launch a series of attacks against a local fort and earn their commonly known name, Tusken Raiders.
...the new colonists seem to have been unaware of the Sand People, but a series of attacks between 98 and 95 BBY forced the abandonment of Fort Tusken, and from that point on, the Human settlers of Tatooine referred to the natives as "Tusken Raiders".
So while you might have meant to be facetious when asking if shooting at the local racers was a fun sport for lazy Sundays, you may have actually been closer to the truth. That is, they were living up to their name of being Raiders and were harassing settlers.