Staying at 1500 RPM allows the engine to go into and stay in DFCO (deceleration fuel cut off). This means the engine is not using fuel, and the reason it can feel like you are slowing down when going down hills.
As much stuff they did to the ECO to make it what it is, they could have explained DFCO for each gear and what situations create this state.We could only wish that DFCO was explained in the owners manual. It's not. You came to the right place to get the answer - even your dealership probably won't know why the Cruze does this, which is unfortunate.