Because the framing of what is the standard or default matters in determining whether a problem needs solving at no cost or merely needs a solution to be available in the market.

If the standard is everyone can choose whom they sit next to (assuming seats are available), then parents are at no disadvantage. This is how air travel was for a very long time, when tickets were much more expensive and much more all-inclusive.

Now, people are seeking cheaper tickets, so the airlines propose to offer discounts for passengers to forgo some of that all-inclusive nature and if those forgone items are a good match for your needs, feel free to take advantage of them. If they're not, feel free to buy a ticket that meets your needs.

No one would think that when the USPS offers Next Day Express, Priority, and Parcel Post that a parent should get Next Day Express for the price of Priority or Parcel Post just because they're mailing something for their kid, right? When a rental car company charges a family of 6 more for a large car than a childless couple is charged for an economy car, are they violating some kind of social contract? "Use discount code BUTIHAVEFOURKIDS to rent a Suburban for the price of a Civic." A landlord charging more for a 2 BR than a 1 BR also hurts parents, but I assume most people think that's logical and proper.

> we generally want to encourage parenting and reduce the burden on them by using the power of the state to spread such costs out

Some people want that. Not all people want that and probably no one wants it in unlimited amounts. I have kids and I'm largely indifferent on the topic beyond supporting strong K-12 public education. I do observe that some people take the notion of "we should spread out the costs of kids" way, way too far for what I think is rational.

Selfishly, I'd be perfectly fine if Basic airline tickets were made illegal for everyone. It just makes my looking at airfares online more annoying because I'll never buy a Basic fare. But, people who do find Basic fares to meet their needs ought to be allowed to have access to them, so I don't actually want them banned.