I think the major and the obvious difference between a cell phone company allowing it's users access to the 911 system and the SPOT system is that SPOT is selling a service where their people contact the authorities on the behalf of those using the SPOT device.

If a cellular phone company offered to call EMS for me using their operators and I tell them I'm in the midst of an emergency and to please send help and they do so, they are acting as my agent. If they do so without ascertaining what the emergency is, then they share the liability. In the cases where people are using SPOT for what I think most people would call frivolous reasons, it was the good people at SPOT who actually called for help. If they're going to use publicly funded EMS and 911 services to provide help for their customers, then they should be held liable for the misuse of their own system.