If your phone is rooted, there is a possibility that you have software and mods that are not supported by Sprint. Why should they waste resources on trying to troubleshoot something that is not within their supported scope? It would be like asking an Adobe Flash dev to troubleshoot Microsoft Silverlight
Honestly, you know when you root your phone what your getting into. And there is a really really good chance that if something software is broken, its probably your fault for running non-stock software. So why would you expect them to support that?
Heres my rule of thumb, and I run Rooted with custom roms. If I have a problem with my phone that I think may even be remotely related to software (battery charging for example), I flash back to stock and run it for a few days to see. If the problem is still there, then I take that stock rom'd phone in for service.
If you decide to root your phone and run non-stock stuff....be your own support tech first. If you cant fix the problem yourself, return the phone to OEM, and see if the problems still there and then whine to Sprint.
Its real similar to what I deal with at work. Our end-users will bitch and moan till they get "admin rights" on their desktops. Then what happens? They run out install google toolbar, and then come crying to us because none of the companies internal intranet sites are resolving.