Because Baseball has a special Congressional anti-trust exemption due to being the National Pastime.
-PJ
The anti-trust exemption was common law made up by the Supreme Court in (I believe) Federal Baseball Club v. National League, 259 U.S. 200 (1922). The opinion was written by Holmes. Congress specifically did away with this common law exemption in the area of labor relations with the Curt Flood Act of 1998.