In the United States, the jock tax is the colloquially named income tax levied against visitors to a city or state who earn money in that jurisdiction. Since a state cannot afford to track the many individuals who do business on an itinerant basis, the ones targeted are usually very wealthy and high profile, namely professional athletes. Not only are the working schedules of famous sports players public, so are their salaries. The state can compute and collect the amount with very little investment of time and effort.
Example, after the 2000 Major League Baseball season, Alex Rodriguez signed what was then the largest contract in American sports history, a ten-year contract worth $252 million, with the Texas Rangers. The tax collecting authorities of other states were notified alongside the public, and would separately demand that Rodriguez's employer withhold the tax due from his salary and remit it to each of them. Even though the state of Texas did not have an income tax, he still had to pay the various state income taxes applied to each away game in each location. It is estimated that Rodriguez paid $520,000 a year for state income taxes outside his own state.
Why does Superman stop bullets with his chest, but ducks when you throw a revolver at him?