The issue with the Sherman Act is that it is broad and sweeping and leaves discretion up to the government.
So the answer to your question is: monopolization is illegal according to the the Sherman Act, and the power to regulate markets is well within the (extremely broad) scope of existing laws (including the Federal Trade Commission Act and the Clayton Act).
I agree with the sentiment about media sensationalism but I don't think "space junk" is an issue we should be so quick to dismiss.
Just because the problem hasn't gotten too bad yet doesn't mean it isn't real (especially because the problem involves a positive-feedback "domino effect" and junk might be infeasible to clean up).
With regard to Starlink: I was a little bit concerned because of the large number of satellites they're putting up, but as far as I understand, Starlink isn't actually a big risk right now as far as space junk goes because the satellites orbit at low altitudes (340 miles) and are expected to naturally de-orbit relatively quickly due to atmospheric drag. (The decision to put them at low altitudes had to do with network latency.) If Elon were to say tomorrow, "hey we're moving Starlink out to 500 miles" I would hope some kind of regulation would be able to prevent that.
Overall, I'm happy they're actively involved. The hands-off attitude in the US is terrible.