This change should not come as a surprise. Now that UA is installing a premium economy cabin on their longhaul fleet, they need to offer those travelers a Y+ seat on the domestic segment. It is more of an incentive to purchase a PE ticket PHX-IAD-BCN, for example, if the PHX-IAD segment is in Y+ verus Y-. Now, UA could change their poilcy to just allow "Y" and "B" fares access to the Y+ cabin at the time of booking (as "B" is the booking code a PE ticket uses for the coach segment). And IIRC, that is how it was when UA first introduced Y+. But doing it this way allows UA to generate more revenue by selling a Y+ fare outright using different fare basis codes, and for a lot less than a normal Y or B fare.