Reuters makes it clear....

The law is that you can't do any pro or con campaign ads... OK that makes more sense. The other hook is the insidious Canadians interferring with our election process:
"Citizens United also said the ads would violate rules against foreigners engaging in U.S. election related ads. One of the film's distributors, Lions Gate Entertainment Corp., is Canadian-based."Top News Article | Reuters.com

Give me a ficking break...