After an athlete spends his or her whole life training to win an Olympic medal on behalf of our country, our country thanks them by handing them a big fat bill.
All prize winnings, from state lotteries to black jack wins to Olympic medals, are considered income by the U.S. government, and therefore are taxable (even if those prize winnings pretty much mean sacrificing your whole life to bring honor to our country).
And it really adds up. When an athlete medals, they don't just get a shiny round thing to bite in front of a camera, they also get a cash prize: $25,000 for gold, $15,000 for silver and $10,000 for bronze. So, depending on which tax bracket that athlete is in (based on the money they make during the rest of year), that could mean they're writing checks to Uncle Sammy to the tune of:
Gold: $3,750 - $9,900
Silver: $2,250 - $5,940
Bronze: $1,500 - $3,960
So, assuming that Michael Phelps is hanging out in our top tax bracket, he currently owes $55,440 for being a national treasure.