On the face of it I would agree with you 300 hours P1 for an SEP is a long way over the top ( I regularly fly with first officers on a B738 who don't have much more flying time) however I think that the 300 hour restriction reflects the fragility of the new generation of (very) light aircraft, they are built down to a weight and therefore are much more easly damaged by even moderately poor handeling.
I suppose on that basis it could be argued that a low hour pilot who is much less set in his ways will be easier to train and come to terms with low inertia and fragility issues than a hardened old veteran with hundreds or thousands of hours on a "conventional " SEP.