Just by way of historical context, let's remember that it was good old Delta that introduced the concept of "basic economy" fares, and it was the other lagacies that fell in behind like a bunch of good little duckies/lemmings.

They did this as a means of countering Southwest and the ULCCs that were/are always nipping at their heels. I'd argue that Southwest still has the most family friendly policy, what with family boarding between A and B groups and "Seat Saving".

My interpretation of the United policy is that while E+ and First are specifically excluded, what this means in practice is that one family member cannot purchase E+ or first and expect United to seat family members together in this class. I may be wrong, but it still means that at the 11th hour United could upgrade family into E+ (My guess is that they won't be upgrading them into first). I call this a counter to the "Southwest effect" in which one family member purchases "Early- Bird" and save seats for the family.