Perhaps there was confusion from when they first started rolling out PP seats. In the very beginning, they had some planes with the PP seats installed before they were actually selling PP seats. In that case, it was possible to simply select the seat because it was not being treated any differently by the system.
Now that they of rolled out PP, you can't do that anymore.

Another point of confusion is the difference between Premium Economy, Premium Plus, Economy Plus, Preferred. "Premium Plus" is UA's brand name for "Premium Economy" and is sold as a different class of service. Everyone has to pay extra to get these seats. However, Economy Plus, Preferred, and Economy are in the same class of service. Economy Plus gives you more leg room. Preferred are a random assortment of seats (perhaps closer to the front) that they choose to charge extra for. If you have status, you can select any of these seats, otherwise you usually have to pay extra.

The only exception is if there is some extenuating circumstance where they upgrade you to meet operational needs. (flight overbooked)