There are two types of roaming actually.
The one that shows up on your phone is the regular type of roaming and counts as roaming. Meaning it uses your Anytime minutes instead of Any Mobile Any Time, will get you bumped for excessive roaming, etc.
The second one is a Strategic Roaming Agreement and does NOT show as roaming on your phone and is not considered as roaming on your account. It looks just like any other Sprint network call.
With regular roaming, Sprint pays the other carrier when you use their towers. That's why Sprint would rather you not roam.
With a SRA, the carriers allow each other's users to use either network. There may be some compensation one way or the other depending on what was negotiated. This is what happened in Wyoming and other states in that area that are now roaming to users. The SRA with Alltel ended. Sprint had few if any of its own towers there as it was not worth it for the number of subscribers. They just made a deal with the local carrier. That's over.
I suspect we will see more of this while Network Vision is being deployed. Sprint has more towers that any other carrier, but they are split between Sprint and Nextel. Once Nextel is finally killed off, you will see significant native coverage across the US.