What is your real evidence that the split-finger FB is a source of arm injury?
Pitch count recommendations for youth can be correlated to actual research that was done at the ASMI by Flesig, Andrews, and others. They found a quantitative relationship between youth pitcher over-use and arm/shoulder injury rates, but they failed to find any relationship between any specific types of pitches thrown and increased injury rates. They actually looked for specific evidence that the curveball was injurious at youth levels in their study--but their results didn't reveal any such correlation and they were honest enough to publish the data as it was, not as they wanted to see it.
The ASMI group did speculate, however, that youth pitchers with outstanding FB velocity would automatically get used more by the coaches of their teams--thus, they seemed to conclude that it is likely that early success with the fastball may lead to a higher chance of injury.
IMO, the "splitter is injurious" advice is no better supported by any real evidence than, "you should never throw a curveball until you're 16". Or 14. Or whatever.
Perhaps more to the point: Pitchers' mechanics should be carefully looked after by knowledgeable adults, youth pitchers should condition themselves properly for their sport, and pitchers should learn how throw each of their pitches correctly. Furthermore (and this seems the hardest thing for coaches to resist) as youth pitchers become really good by optimizing mechanics, conditioning, and control of each of their pitch types--coaches (and pitchers, and their dads) should learn to avoid the temptations of 9-13 yo glory. Pitcher over-use at youth levels is not a pretty sight.