Is this a per-market thing?I’m from Chicago and therefore fly United with my family all the time. The website/app lets me pick all our seats at booking time in Economy class without any up charges.

It's a benefit of "Economy" vs "Basic Economy". I saw it on an international flight. You pay 20% more and are allowed to sit with your family. At least that's how I understood their marketing. There also seem to be some exceptions for kids under 12, but I'm not sure how they work.