Basic Economy fares take the lowest fare being offered by an airline, and work to give customers less. Delta started by restricting changes and seat assignments. United and American doubled down by saying that whereas you can even pay for a carry on bag flying Spirit Airlines, they won’t let you bring one on at all.
On the same day that American Airlines went live selling Basic Economy fares on ten routes, United has also announced the sale of Basic Economy between Minneapolis St. Paul and United hubs.