Why does an iPhone need to be unlocked for T-Mobile?

Title explains it all. In further detail, I'm wondering why an iPhone needs to be unlocked for T-Mobile unlike AT&T, Sprint and Verizon. And will T-Mobile ever have the low prices like the 3? This question has been irritating me for so long.

It doesn't anymore. At the time it did because T-Mobile was not an official iPhone carrier, but now that they are one, you could go into a T-Mobile store & buy one locked to them & go on.

No, this does not mean that older iPhones suddenly are able to work on T-Mobile despite being locked. They reflect the time that they were not a carrier still & will forever.