Rather, it's to ensure they do not exercise market power to the detriment of the consumer
Yeah. We sometimes forget what a nation is actually supposed to be: It's a bunch of people coming together to form an entity that can do things individual people can't do, for every person's benefit. We can't all build our own little streets, it makes more sense if we all pay a bit, and a larger entity builds a consistent system of streets for us. Likewise, we can't all enforce our own law, so we come together, come up with a law most people can agree with, and pay for a police who can enforce it.
Democratically elected governments are supposed to make our lives better.
Often, that goal aligns with a free market. We all tend to profit from free markets. But sometimes, it doesn't, and when it doesn't, we shouldn't assume that a free market is somehow a goal of its own; it's merely a tool to be used when it is in our best interest.