Its a brake saving feature. Downhill breaking, especially long hills, creates excessive break heating and wear.
If the transmission upshifted you would speed up from reduced engine breaking. If you press the gas to upshift, you would speed up also.
So the question is, do you want to speed up? If you do, press the gas. If you don't, let the engine and transmission help maintain a lower speed. Revving the engine is not wasting gas since the throttle is closed. I suspect it will upshift if RPMs are high enough. Use the brakes to control this on steep grades.
This behavior is desirable once you get used to it. My Super Duty Ford operates this way.
2009 Murano LE AWD - Traded
2003 Excursion 4x4 7.3 PSD - DIESEL
1985 Mercedes 300SD - DIESEL
JGC LIMITED 4x4, CRD, MAXSTEEL-BLK/FROST INT, LG2, ORA2, BLK HTR, NAV., CD - DIESEL!
Harley FLSTFI '04