Most phones can be unlocked. For an iPhone, the difficult part is to jailbreak it and doing it yourself can be a little painful, but that hasn't been a problem for many people. So somebody stops paying their bill, or declares bankruptcy. Do you think there's anything stopping them from unlocking their phones for pennies?

As an anecdote, I once stopped paying my bills to Vodafone and gave up on my contract obligations. At that time Vodafone demanded a 250 EUR interruption fee, which in the meantime became illegal in my country. Well, I told them to dare and sue me, because I have proof of unjust treatment and service disruption on their part and I did have that evidence. Basically they were assholes, they knew it and they gave up on any claims. I also unlocked my iPhone 3GS by myself and moved my business somewhere else.

Just like DRM, there is absolutely no valid reason for selling locked phones, other than lock-in, i.e. to prevent you from using other carriers while in roaming and thus suffer roaming charges. But even that is just annoying, because you can get unlocked feature phones for $30 without a contract, with a battery that lasts for a whole week and just use that.