Imagine a law that says "you're free to speak, and spend money speaking, but if you mention an elected official, you might get prosecuted for corruption." How is that free speech?

I am. I'm trying to illustrate why that opinion of the court is not actually absurd or insane.

You might not like its implications, but there are other, worse, implications if merely speaking about a subject (e.g. a candidate) can be considered a form of corruption.

If money is speech, then how are any constraints on campaign spending possible? But the Supreme Court has maintained some constraints, so it's a practical judgment call, balancing free speech against the potential for corruption. They didn't have to draw the line where they did in Citizens United.

Of course there can be restrictions on campaign spending (i.e. classical bribery). Free speech has never been a defense to crimes like bribery, fraud, etc., that incidentally involve speech. But the situation in Citizens United came nowhere close to that. It was core political speech. The money wasn't a campaign donation, it was used to create and distribute a political movie about a candidate. If the Supreme Court didn't draw the line there, it would have obliterated the line.

Citizens United was 5-4, yet it was a sweeping decision that obliterated several decades of election law and bent political advertising expenditures to match the accelerating inequality of wealth distribution in the US. The world would not have ended if, as was originally expected, the court had made a more narrow ruling.

There are no constraints on campaign spending. Trump spent like $60 million of his own money on his presidential campaign.

There are limits on how much money any individual may give to a campaign that is not their own, and a prohibition against organizations (corporations, unions, etc) giving money to a campaign.

Why? Because there is a difference between spending your own money to enable your own speech (independent expenditure, the subject of Citizens United), and giving your own money to someone else to spend however they want (direct contribution, whose limits Citizens United did not disturb).

Asserting that independent expenditures don't cause "harm" in terms of corruption or the appearance of corruption is arbitrary. In fact, independent expenditures are quite likely to lead to excessively favorable treatment even in the absence of an explicit quid pro quo.

Limiting campaign contributions is a limit on speech, and limiting independent expenditures is also a limit on speech. Those that insist that one causes harm and the other does not have merely arrogated to themselves the right to define "harm".

It's not arbitrary, it is a logical necessity to protect free speech. Otherwise political speech exists in a murky gray area where it's "free" unless some prosecutor decides that it's actually corruption. There's your dangerous arrogation--which thankfully the Supreme Court preempted.