As the article states, many of our networks already sell their handsets unlocked. 02 before they sold handsets unlocked would offer you the chance to unlock your handset free of charge. They still offer this for handsets that have been sold previously to their new no locking rules.
If you get a contract and fail to pay theirs many ways they can get you, and in fact locking a phone was never a way to prevent this as people would be willing to buy and sell a phone which was locked to certain networks, including the main resellers.