Huh? E+ is not a class of service at all. It is widely known that it is not protected. UA freely sells tickets when no seats are available, noting that seats (in E+, often) well be assigned at the gate. Of course that's the policy.

This is a bit like the twilight zone. Lots of airlines charge for advanced seat assignments and guaranteed good seats in economy. They all give those seats away for free day-of to folks who didn't want to pay for the guarantee. It's totally normal. The only thing that wasn't was this particular agent's approach to determining who got the seats. They were probably just trying to be nice, but I agree that the approach was bad.