No, I don't think it's contradictory. Money can be speech, but sometimes there are good reasons to restrict free speech. One reason is that money can corrupt the democratic process--which is why the Supreme Court hasn't struck down limits on direct contributions to candidates (but god help us if they do).

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.