Different type of corruption from what you’re probably thinking of.
Below the level of the Presidency, which gets so much publicity as a matter of course, campaign finance restrictions are structurally going to favor the incumbent (of either party) because the incumbent has the advantage of name recognition and the public nature of the office. A challenger has to spend more to overcome those hurdles, and requiring them both to the work under the same limit can prevent that from happening.
This may be different from a parliamentary system like yours, where name recognition is relatively less important because of the strength of party identity. In your system, voters generally know what they’re getting from a Liberal candidate, for instance. Down here, simply saying “I’m the Democratic candidate” carries less information, without further explanation.
I’m not dismissing the concern about not letting people with money tell you how to vote and choose who you will vote for. I’m just pointing out that there’s a countervailing concern about not letting people with power do that either.