I agree. This would be a strategic decision since the iPhone is more popular overall. I think Sprint wants/needs an LTE iPhone and wants or promised to get enough LTE markets up by this fall so Apple will be convinced to include Sprint in the rollout. Since LTE wasn't ready, I don't believe Apple would invest the resources into an LTE iPad.
Check me if I'm wrong, but Wi-Fi only tablets are still the biggest sellers. 3G/4G models supported by carriers haven't been knocking any doors down in sales. My guess it's because you have to pay for an extra plan that no one wants to buy. I think if the bucket plans like AT&T and Verizon are contemplating, if done right may help out.