You can't really use HAM with dynamically animated objects - the backgrounds may be static but the characters and so on are all animated. The colour of each pixel depends on the colour of the previous pixel, so you'd get streaking all over the place as the characters moved around in front of the background.
It would have been cheaper to dynamically alter the palette with the copper depending on the colours required for each scanline. Even in low-res 5 bitplane mode the A500 isn't limited to 32 colours - the copper can be used to change the contents of the colour registers as the screen is drawn. But Lucasarts weren't really making much use of the Amiga chipset, it was like you say just a cheap PC port (shovelware).
