The pcm senses the load on the engine and reacts to it faster than we do. When the cc is on the pcm is trying to maintain that speed as best as it knows how and being that shifting to a lower gear is better than applying 3/4 throttle it stays in the lower gear. When you are towing and not using the cc we react to speed changes slower than the pcm does so we may not force a downshift.
Another good point to mention is that when towing at higher speeds in higher gears the clutches inside of the transmission tend to slip more which leads to heat buildup and shortens clutch life. By having the transmission in a lower gear the clutches don't slip as much and the trans stays cooler.
On my Tundra I have tow/haul mode for the trans. When I put it in tow/haul mode the trans will usually stay in 4th gear and only hit 5th or 6th on very flat road when I am towing my toyhauler. If I don't engage tow/haul mode the truck will stay in 5th gear more, the trans will shift between gears more, the trans temp rises about 20 degrees and I get the EXACT same mpg as I did in tow/haul mode running in 4th gear and turning 500 more rpms.
