Hi Bigrob.

This has started happening a lot more lately.

If you didn't pay the $40 and just waited at the gate, you'd have been assigned any available seat either in E- or E+.

So, your $40 would have gone to pay for:
1) More legroom in E+
2) Being able to choose your own seat rather than being randomly assigned a seat somewhere.

This is what UA's new strategy is now... let's say you have a flight with a total 120 seats. 70 seats are E- and 50 are E+. UA will sell 120 seats. However, if you don't have elite status and the 70 E- seats are all been selected, you get placed in a "seat assignment at the gate/checkin bucket."

The idea is to try to do an upsell to the people already seated in E- and to those without a seat assignment. Some agents at checkin have been a bit slimy and tried to convey the idea that if you don't pay the extra $, you won't get on the flight.

However, you do have a seat on the flight. If some people in E- take the upsell, and get E+ seats, the gate agent will assign you those people's former seats in E-.

If nobody in E- buys the E+ "upgrade" then the gate agent will seat you somewhere in E+.

Any particular reason you didn't get a seat assignment when your ticket was purchased?