Not really sure what your issue is - likely you purchased near travel time. The seats available for assignment prior to check-in for a non-elite were already taken. Every airline reserves seats for airport assignment. The airline in the interim pushed buying an E+ seat which could've been assigned during the period approaching travel, but you didn't want to buy. It's a business - makes total sense.
When you got to the gate, they assigned the seats still available which included E+ seating - you got a better seat than you purchased for no additional money.
Just to be clear - you realize your ticket didn't entitle you to E+, right? Because it absolutely did not - full fare or not!