I've looked into this in the past; I still believe it's technically legal on GMRS if you want to argue it. I took this up with the FCC and below is what I received back from them. I think there's grounds to argue against their response, but it settled the issue for me (for now) anyways. Justified or not, and while APRS as a protocol is allowed, their stance is that they don't want it unless limited to a certified low-power handheld radio with fixed antenna, which severely limits its application as it eliminates the ability to develop a solid digipeating/gateway infrastructure (which I had planned to start developing at the time on one of the 462 interstitial 'FRS' channels)
My original request to them:
Their response: