A number of years ago now, there was a significant change to how cell phones were handled; the act of locking a phone and its Sim Card to a certain carrier was effectively ended.  At that same time there was another part of the phone being locked and the major carriers used lies and fearmongering to keep that part of the phone locked, preventing the actual owners of the phones from being able to make full use of the devices which they own; the bootloader.

Today, the majority of the world can buy an android smartphone and after a short period of time, if they wish, they can unlock the bootloader.  However, people here in the United States (and a few other places) can buy a phone of the same name but these phones almost all tend to have permanently locked bootloaders.  The only thing that this does is prevent the user from being able to fully customize their phone, improve its performance and efficiency, as well as greatly lengthening its life.  They cannot customize their phone fully because the carrier version of the OS regularly locks features and limits what the user is allowed to do with it.  There are numerous ways to tweak an OS so that it will perform more efficiently, the Android OS is even built with the idea of it being customized and users building their own flavors, similar to it's parent OS Linux.  The biggest thing that locking the bootloader does is force the users to buy a NEW phone sooner.  A phone with an unlocked bootloader can have its stock OS replaced with a custom one.  These custom OS roms regularly tend to perform better than their stock OS rom brethren, and it is also typical for the custom OS roms to provide major version updates to phones which are 5 or more years old.  So phones which the carriers have not updated for 2-3 years or more can still be getting the newest versions of Android even if their phone never did officially., making it much easier to keep older phones like that.

These restrictions implemented because the makers of the phones wish it, but instead because the Carriers demand it.  This can be seen by looking at the International versions of the various Android flagship phones like the Galaxy S series from Samsung.  The non-domestic versions of these phones have the option too OEM unlock the phone, that is to unlock the bootloader after having had the phone activated for a short period of time (typically seven days).  However the US versions  of these phones do not allow such options to work.  They have made claims that it is to keep their networks secure, protect their customers from themselves, and being overwhelmed with people calling the carrier's CS department with issues caused by doing something involving a non-stock OS.

The first of these is an empty claim, they have never produced any evidence that any such danger was or has ever been introduced to a network due to the bootloader being unlocked.  The foreign carriers don't have any of these detrimental effects, and they have just as large if not larger market.  For that matter, in almost all cases, these same carriers allow Windows and Linux computers onto these same networks without restriction, and there is much more likelihood of those devices being capable of causing major damage to a network.

Next, there is nothing that they are doing which protects their customers.  Your average user would not even know that the phone's bootloader could be unlocked, let alone consider doing it.  Then even if an unknowing user did get to the point where they were to unlock the bootloader without realizing it they still would not be able to do anything much without having consciously deciding to make major changes to their phone.  At that point, the carrier is not protect its customer, it is handicapping them. 

The third claim that having the bootloader unlockable would result in the carrier's CS department being overwhelmed by calls asking for help regarding it is ludicrous.  As mentioned earlier, the majority of end users won't even know it can be done or what it would do.  In addition, the people who actively make the effort to unlock their device's bootloader don't use the carrier's CS unless it has to do with the actual service.  A person who consciously chose to unlock the bootloader and do something like root their phone typically will inquire about problems with others who do the same.  They will search for people with the same issue and work together to resolve any problems themselves.

The carriers should not be allowed to require phone designers/manufacturers to lock the bootloaders for them.