I don't think that's an oddity, that's very much a calculated move. The original APK included in /system had the Sprint button. Because this is a pure el goog phone, a silent market update occurred after the first time you opened the market placing a new APK in /data which removed the Sprint button.
If you go to Menu > Settings > Applications > Manage Applications - under the download Tab you can see "Market" is listed (FYI - The download tab is /data). Click that, click uninstall updates, open the market and I'll bet the Sprint button is back but it's downloading the silent update again in the background so if you close it and wait a couple min you're right back to having the new market.
EDIT - I'm not saying the market won't have a Sprint button in the future but i highly doubt it and google would have to approve such a move considering they own the software for the phone, not Sprint.