WARSAW—Poland’s President Andrzej Duda said on Monday he vetoed a controversial media bill that would have forced U.S. company Discovery to give up its controlling stake in Polish television network TVN, with the president acknowledging free speech concerns of the bill’s opponents and suggesting it would have strained relations with the United States, a key Warsaw ally. Duda said at a Dec. 27 press conference in Warsaw that, while he backs future legislation that would curb the ability of foreign capital to control Polish media outlets, the current bill’s retroactive impact would be unfair to investors and would violate a U.S.-Poland trade treaty signed in the 1990s. “If we signed an agreement, we have to abide by it,” Duda said. “Then we can say we’re an honorable nation. And that’s how I want Poland to be seen by its allies.” The bill, recently passed by the lower chamber of the …