Apple intends for locked iPhones to only activate when a SIM card from the providing carrier is inserted or no SIM card is inserted - this doesn't have to be the iPhone's original SIM card and it doesn't have to be a SIM with active service, it just has to be a SIM that is valid for that iPhone. The relevant information about activation is that when an iPhone is booted up for the first time or freshly after a full restore, the phone's serial number and IMEI are checked against a database in Apple to make sure it's allowed to work with that SIM card inserted (this also activates push notifications to the iPhone). Hacktivation is a way to using an iPhone without an authorized carrier, or when bypassing iCloud Lock. Hacktivation is the process of circumventing the standard Apple activation process with iTunes.
