Depends for example Apple might request that there new phone is locked to a certain provider for a fixed amount of time so whilst another handset for example by Sony wont have the same conditions. Check before you buy a phone if it will be locked or not.
I am not sure you have got the right info. I have just bought the new iPhone 7 Plus but because it is part of a 'plan', the phone is blocked (to be used for BT only)!!! But if you buy the phone outright via the BT Shop, strangely the phone is not blocked! So, you need to know if you are purchasing the phone via a plan, or just the phone itself without any plan.
if you buy a contract iphone 7 with any provider it has to be locked to that provider for 6 months before it can be unlocked that is a requirement of Apple.