You are forgetting about keeping the cat warm - at high coasting speed the cat is being cooled off and the engine idles faster to warm it up. If you stop it warms up - I think this is what happens to my xB - I stop at a stop sign for about 20 - 30 seconds and the idle drops to 650rpm from 850rpm.
Because at 65mph the tach would lurch to 2100rpm whereas at lower speeds of say 30mph, there is basically no lurch because shifting into 5th gear from neutral at 30mph means the rpm would jump to 1050rpm. No reason to have the idle be as high as 1000 at 30mph....
Well, that explains more thoroughly why it would second-guess the driver for that type of shift at that speed, but my question remains...why wouldn't it second-guess the driver for the sake of smooth shifts during OTHER types of shifts?
If the ECU programmer had the foresight to know that the driver would neutral-coast at 65mph and then shift into 5th...he would also know that the driver would neutral-coast at 30mph and then shift into 2nd or 3rd. Sound weird? Well, so does neutral coasting at 65mph, if you're not a hypermiler. Much more normal, very common and done by everyone, is downshifting from 5th to 3rd to accelerate (often for a pass); I would expect it to second-guess the driver that time if it's going to do so for other times. Why put in special programming for something people will never use, but then not put nearly the same program in for something everybody uses?
The question of keeping the cat warm, that one makes the most sense yet; it's essentially a "cold idle" behavior. Let's compare it to cold idle... Does the car idle higher for a few minutes when it's first started (but not driven away), or just for a few seconds?
Here is a picture of mine, sold it 3 years ago. Got better mileage than my Tundra with a V6. All original 50,000 miles, still 6 volt. Adjusted the mechanical brakes with a digital thermometer, measuring temps at each wheel.
I guess they didn't vary between cars and trucks under the hood. The picture I posted was from a truck. I'm wanting to take one of those flat-head v8 Ford engines and slap it onto a motorcycle. Sounds cool and not a suicidal amount of power.
I had a flat head 6 in my first car - 1965 Rambler American - talk about a smooth running torque generating engine only mine had a rebuild after my first summer and ended up with slightly higher compression(195psi), head polishing, SU carburation and water injection to keep the carbon from building up.