Earlier quoted context omitted.
Why not do what is done in the UK and put spending limits in place?
Because ultimately a spending limit boils down to "you're not allowed to air this film" because you already spent too much. In fact, that's exactly what the Citizens United case was. We have a Bill of Rights which guarantees that Congress shall make no law abridging the freedom of speech, the right of the people to peaceably assemble, or the right to petition the government for a redress of grievances.
It drives me crazy that people don't understand Citizens United was about free speech. And most people who say they don't like the ruling reverse positions once I explain exactly what it was about. Stopping a film maker from making or airing a political film seems horrible to me.