I would speculate that this "feature" may be for emissions testing or other diagnostics where the engine may need to be above idle speed. It also serves to show that the engine is running. Only situation I've seen "force charging" (don't have a gauge) would be brake on, flooring it at a light, then letting off for a high torque launch. You'd probably have a negative battery draw there as well though.
Check this out... (Click on the "THS" to enable THSII mode). Drag just the engine control up. Maximum revs while stationary are 2775 (otherwise MG1 will exceed 10,000 rpm) http://homepage.mac.com/inachan/prius/planet_e.html
