This is pretty standard now in the airline industry. I get the frustration if you don't qualify for a "premium" seat for free, but it's typical of most airlines.
As noted, if you don't have a seat assignment you will get one at check in (unless flight is oversold) and it could be in any of the seats - and often is economy plus or preferred because a lot of fliers don't want to pay extra for those leaving them open until flight time. (I have been on several recent flights where it looked like there were >50% of seats still available but the flight was, in fact, fully booked.)