without carrier permission
Which currently, all major carriers provide once the contract has expired.
The real problem is that the normal monthly cell phone plans that the carriers have provided for the last 10+ years all clearly are priced high enough to cover the cost of the subsidized phones everyone is getting, and you had to pay that artificially high rate even if you purchased an unlocked phone at full price. Obviously, the subsidy is the main motive the cell companies have for not letting you unlock your phone and go to another carrier.
In the last couple years, more and more carriers (Virgin mobile, Smart Talk, T-Mobile) have begun to offer cheaper monthly plans that do not include a subsidized phone. You'd probably end up paying a similar amount if you kept buying a $599 iPhone every 2 years, but this is a much better model as it a) will save a lot of money for people won DON'T buy a new phone that often, b) shifts the prices of these goods and services more into line with their actual costs, and c) precludes all questions of unlocking or other carrier restrictions since the phones are being purchased at full price before you even sign up for a plan.