It probably is about the arb clock, but that's not ridiculous. By waiting until early June the Reds keep control for an extra year. If they bring him up now he'd probably qualify for a super two - I, for one, am in no hurry to see him go the Yankees/Red Sox/Dodgers. Its probably a consideration for Homer's callup also.
I'd much rather see the Reds top prospects developed enough in the minors so they can contribute immediately rather than spend the first few years under Reds control doing their development at the Major Leaque level.