The phone is locked so that it can be used only with a specific SIM. So, the SIM is the key to make the phone useable. Therefore, a SIM-lock (well it should be called SIM-key).
I doubt that. It seems more likely that the phone is locked to SIMs that are for the network of the carrier that subsidized it. Think about how much manual labor and careful matching would be needed if it were to a single SIM. See also http://en.wikipedia.org/wiki/SIM_lock.
