They weren't fully illegal at the time. Citizens United actually got the idea to do it from Michael Moore's film Fahrenheit 9/11, which was theatrically release

They weren't fully illegal at the time. Citizens United actually got the idea to do it from Michael Moore's film Fahrenheit 9/11, which was theatrically released in the small window between primary and general election seasons but advertised and shown at Cannes during the primary. The FEC's policy was based on a standard of "bona fide commercial activity" - filmmakers were covered by the restriction if and only if the FEC didn't think they were trying hard enough to make money. Of course, like you said earlier, maybe they would have been fully illegal if the US didn't have a written constitution requiring lawmakers to think in terms of principles. It's a hard hypothetical to reason about.