If there is no input profile available, because you are scanning a negative, or because you don't have IT8 calibration, there is no actual conversion into ProPhoto, because no reference is available for the input data. The output is being tagged as ProPhoto and this results in the highly saturated images.
The above statement from LaserSoft needs clarification. The way this reads to me is that there is
conversion into ProPhoto when scanning negatives provided one has calibrated the scanner with an IT8 target.
If this is true, then this would be a good reason to calibrate one's scanner.