Good question.

Government should represents and advocate for the best interests of the entire population. It should legislate for both people and companies to behave fairly towards each other.

Consider the gym membership. You join and sign a one year contract. The terms are that you can cancel after that period, but if the gym hides or dark-patterns the cancel procedure, are they really offering you the contractually agreed upon ability to cancel? They are not.

We all know what fair is. Just because companies (and sometimes individual people) have outsized power against their customers, it does not give them the moral right to abuse that power.

So, I believe in this case Mamdani is engaging in both good and well-functioning governance. And that to me is morally legitimate.

Not every law is legitimate. Governments used to be formed legally by a hereditary succession, was that legitimate?