If there was a way to opt out of exposure to political advertising (kind of like can-spam and do-not-call lists), and most Americans adopted this option, maybe it would remove the motivation to exercise this 1st amendment right.
All google has to do is 'give' x ad views before some videos on youtube, for each candidate. If someone tries to get Google to do otherwise, they're limiting GOOGLE'S freedom of speech.
That said, if Citizens United is any indication, Google could simply donate the money to buy the placements to a Super PAC, which could then buy the placements for each candidate back from Google. Sort of a silly workaround, but hey, it might be a fun way to subvert the outcome of Citizens United.