Edit: It's a common misconception that Citizens United allowed unlimited campaign contributions to candidates. That's not quite true--it allowed unlimited independent expenditures by corporations, but not unlimited contributions directly to candidates. The Supreme Court held in Buckley (1976) that the limit on direct contributions was a constitutional restriction on free speech because of the government's interest in "the prevention of corruption and the appearance of corruption spawned by the real or imagined coercive influence of large financial contributions on candidates' positions and on their actions if elected to office." That ruling still stands.
Edit 2: Citizens United didn't hold that corporations are people--that doctrine has been around for a century. It held that because corporations are people, they have the same speech rights as natural persons--namely, to make unlimited independent expenditures.
It's fairly clear throughout the ruling[1] that the Justices are not relying on corporate personhood, but the freedom of association argument. It is a media myth that the case was about 'corporations are people, too!'
The trouble is that it has the same effect, and creates the same appearance of corruption. Acausal trade is possible; a candidate and their PAC do not have to directly communicate to end up influencing each other's behaviour.
Note that the "is money speech" question is a total red herring. Citizens United was about a movie critical of Hilary Clinton. It was as core to the First Amendment as it gets. If anything Citizens United, a non-profit organization that exists for the purpose of promoting political messages, is far more entitled to speech rights than Apple, a for-profit company that has little political involvement.
Corrected.