Makes no sense to me either - maybe it's because he isn't a card-carrying PGA pro. Seems like such a feat would validate the concept of awarding him full status as he would be beating the world's best golfers in a prestigious event.
It may take him out of the amateur competitions, but what's to say he couldn't be a pro while attending college - last I knew there was no prohibition for a college kid to earn money before he graduates/drops out.
I think the rational is that, people who are pro’s, tend to play at a significantly higher level than the amateurs.
If they didn’t have this rule, many of the pro’s would stay amateurs so that they could just rack up wins and not allow the amateurs, who aren’t capable of playing at a pro level, to ever win a round.