Air time means "you wanna talk about candidate X on your channel? Sure, you're just going to match every single minute you spoke about him with every other candidate.". That usually excludes legal trouble because that doesn't exactly count as campaigning for said candidate. I feel like you are seeing it as having an allotted amount of time for each candidate, where every single minute is counted and taken out of their time counter, when the idea is more to be to allow the channels to organise how they wish, but ultimately, in a 24 hour period, they have to fit the same amount for each candidate. (In practice, you do that over a week or a month, but the concept is the same).
YouTube time is different because there's not one single place controlling your entire feed, noone dictating what you are going to watch. You can decide on your own to watch ten hours of your favourite candidate and ignore every other, because you made that choice and noone imposed it on you.
The Citizens United question is a bit tricky indeed, but two things:
- Ultimately, it is the TV channel's responsibility to have aired such a video, and as such should match any time campaigning for a candidate with others. But, like you said, it was more an attack then anything else. Which brings us to our second point:
- Straight up outlaw such clips that are an attack on a candidate. It serves no purpose other than to drive down the level, it makes the campaign absolutely worthless and is a danger to democracy. It is the most basic level, the dark depth of political life. It kills every debate, destroys every effort made to educate people, because ultimately attacks are cheap, entertaining and efficient. It truly makes society worse as a whole, and people stupid. Yes, that's maybe an attack on free speech. I believe keeping a higher level of debate is a tad more important than being able to take a dump on someone you don't like on live TV.
Let me give you examples of where I live, France, to alleviate some of your fears.
We have a council that ensures what is aired is either nonpartisan, or matched, with the ability to shut down entire TV programs if needed. It has not been done in decades, and most of it ends up being fines. The shitstorm that would happen if they did cancel an show would be quite the sight. Attacks are forbidden. Total campaign funds are capped. Campaign clips are of fixed length, are aired something like four times a day, randomly ordered, shown for all candidates. Public TV has an obligation to give financial and material aid for smaller candidates to ensure they can have a clip. You cannot start officially campaigning on TV and in the press before a set date. You could say we have some rather intensive speech control.
This previous election has seen the two traditional parties (LR & PS) failing miserably. The socialist party, one of the most important parties in the country and previous president's party ended up with a pitiful 6.5% of votes. REM, the new president's movement was born out of nowhere barely a year before the elections. FI managed to crystallize the leftist electorate and reach 19%, barely a percent below the traditional conservative candidate. The FN, as much as I despise them for everything they stand for, is not a traditional party. We have communist candidates. Yes, multiple, because the only thing harder than herding cats is getting communists to agree. We have one that intends to go to Mars. Every election, we have a dozen candidates. Every election, debates are held. They're even rather polite, usually. This year, Fillon has been bogged down in embezzlement affairs, Le Pen too. Aside from one deliciously satisfying bash during one debate, an which lasted 3 minutes over a 3 hour long debate, things stayed civil.
And that's just an example of how we do it here, and I definitely not believe it's perfect. Our political system has issues. Plenty of them. Look at other countries, look at how it's done. Improve on it, and take the best from each.