Imagine a system that forced you to contribute $100 to the political campaign of the winner of each election. In 2016, you’d be compelled to donate to Trump; in 2008, to Obama; in 2000, to Bush.
You’d probably be offended. You might object to the fact that the program gives money to candidates with views that you find disagreeable. Why should a law force you to give money to Democrats if you’re a Republican and vice versa? In First Amendment terms, you’d say the law improperly discriminates on the basis of viewpoint.