All the prime time speakers I have heard have endorsed Trump. The ones who have not as yet (like Ryan) were NOT given a prime time speaking slot.
Maybe Trump is giving the Cruzlims an olive branch by allowing Cruz to speak prime time - so they will not make a mess of things later. No other reason to allow a divisive speech in prime time or any time.
That makes about the most sense of all. Let Lyin' Ted have his moment in the sun. If he dumps a smelly Obama on the stage, it only hurts him, and Trump can always say "Hey, I gave him a chance."