Once the fuel gauge bar thing is on E, the tank still has 2 gallons of gas left?

Was that only if you added a minimum amount of gas (under 3 gallons IIRC), or regardless?

 

Is it not called out on the window sticker, a.k.a. the Monroney label?

Many of us would. Unfortunately, there are enough idiots and clueless and less-situationally-aware customers out there to cause a lot of headaches for the customer support folks -- "your car said it had just enough gas to get me there, but then it ran out and left me stranded in the middle of nowhere on a dark and stormy night!". This is one of several areas where they deliberately design in some 'customer expectation management' to reduce the annoyed / upset / angry customer calls.

The annoyance you experienced from being warned to fill up at 8.x instead of 10 gallons, is very tiny compared to the annoyance of getting caught short due to miscalculation or foul weather in any of the places I pictured here, or in numerous other similar highway segments.