Serious question which didn’t escape the lawyers arguing the case, given that the primaries started in January, and the convention wasn’t until the end of July, how can it be said that their speech rights weren’t unduly restricted for an entire year, given that they would also have been restricted for 60 days prior to the general election (assuming that Hillary would have beat Obama)?

https://ballotpedia.org/2008_Primary_Elections_Calendar

And it is relevant, because the right to advertise Fahrenheit 9/11 was never restricted in this way, because the FEC decided that it was a different sort of documentary. Citizens United even sought to clarify this in 2004.

https://www.rcfp.org/fec-dismisses-fahrenheit-911-complaint/

The majority did decide on broader free speech grounds, on the urging of civil liberty groups like the ACLU, because they recognized that officials in the legislative and executive branches should not have the power to decide that some corporations (like Moore’s) were engaged in legitimate speech while others should be restricted. Or to decide which organizations were legitimate media orgs, and which were political activists, essentially making an artistic determination. Summarized in Wikipedia:

In Citizens United v. Federal Election Commission, however, the majority argued that the First Amendment purposefully keeps the government from interfering in the "marketplace of ideas" and "rationing" speech, and it is not up to the legislatures or the courts to create a sense of "fairness" by restricting speech.

The facts I referenced about the distinction in treatment of the two films are referenced in the annotation of the decision from the court here:

https://supreme.justia.com/cases/federal/us/558/310/#tab-opi...

Of note from Kennedy's decision:

Yet, the FEC has created a regime that allows it to select what political speech is safe for public consumption by applying ambiguous tests. If parties want to avoid litigation and the possibility of civil and criminal penalties, they must either refrain from speaking or ask the FEC to issue an advisory opinion approving of the political speech in question. Government officials pore over each word of a text to see if, in their judgment, it accords with the 11-factor test they have promulgated. This is an unprecedented governmental intervention into the realm of speech.

...

The law before us is an outright ban, backed by criminal sanctions. Section 441b makes it a felony for all corporations—including nonprofit advocacy corporations—either to expressly advocate the election or defeat of candidates or to broadcast electioneering communications within 30 days of a primary election and 60 days of a general election. Thus, the following acts would all be felonies under §441b: The Sierra Club runs an ad, within the crucial phase of 60 days before the general election, that exhorts the public to disapprove of a Congressman who favors logging in national forests; the National Rifle Association publishes a book urging the public to vote for the challenger because the incumbent U. S. Senator supports a handgun ban; and the American Civil Liberties Union creates a Web site telling the public to vote for a Presidential candidate in light of that candidate’s defense of free speech. These prohibitions are classic examples of censorship.

edit: noted the 60-day rule for general election, and added reference to decision.