Far as I know, Either pay Sprint, or well, it just doesn't happen. I know that's not the answer you're looking to hear, but it is the reality.
A more detailed explanation:
When you tether you're not going through the normal data ports, so it can be tracked differently. Even on other phone platforms (Android for example), don't work for tether unless you've gotten that service enabled. The reason the little green robot can do it, is that it's a *nix based platform with the source code available. The developers and hackers have changed the ports back, bypassing the service check.