Some of you may not know that baseball is exempt from anti-trust laws. In fact, Major League Baseball is the only business in America so protected, sport or otherwise, thanks to a 1922 Supreme Court decision featuring Oliver Wendell Holmes. Of course, back then baseball wasn't what it is today - games were local, and there was no revenue-sharing, no TV or radio, nor anything else crossing state lines, except of course for the buses carrying the players. There just wasn't much money in it back then, it was actually all for the love of the game. But I digress.
So the Supreme Court decision may have made sense at the time (emphasis on may). But to date, the exemption has never been overturned, despite numerous lawsuits and legal challenges, most recently in January 2016. And this in part led to to Flood vs. Kuhn in 1972.
Before 1972, baseball has no free agency - players were stuck with their franchise even after their contract was up, so they couldn't switch teams without permission, due to something called the reserve clause. So the Cardinals had every (legal) right to trade Curt Flood to the Phillies in 1969. Trouble was, he refused to go, and he sued Major League Baseball to boot. The odds were against him, as other court decisions prior to this had prevented other players from attaining free agency. Flood was also told that even if he won, he would never work in baseball again, but he persisted.
Unfortunately for Flood, on June 18, 1972, the Supreme Court ruled against him, citing the aforementioned exemption, and ruling that it could only be removed by Congress. As it turned out, however, the case brought so much pressure on the owners that in 1976, free agency became a reality in Major League Baseball.
For a lot more on this story and the events surrounding it, see the HBO documentary "The Curious Case of Curt Flood."