Disclaimer: I only mention the major carriers.
I don't think your statement is true. I think there is a lot of confusion about this and I did some research. Here is what I found, I believe this to be true but could be mistaken.
The unlocked T-Mobile phone is A1549 (6) and A1522 (6+). This is the GSM model and supports GSM and LTE. Both At&T and T-Mobile are GSM carriers and you could easily jump to AT&T using an unlocked T-Mobile phone. This type of phone is the most limited and supports the fewest bands.
There is a CDMA version of A1549 and A1522 which is typically sold by Verizon. This phone supports GSM, LTE and CDMA (a little more versatile). So an unlocked version of this phone would theoretically work on AT&T, T-Mobile, Verizon and Sprint. Although the carriers limit this by "whitelisting" devices issued under their name, essentially blacklisting devices issues under other carriers. I've read lost of conflicting accounts on what the carriers allow. Sprint seems to be the worst and probably for the reason below.
Sprint offers a third version A1586 (6) and A1524 (6+). This model has only been available via Sprint and is ALWAYS sold locked. This model supports GSM, LTE and CDMA but also supports TD-LTE and TD-SCDMA. "You can get access to TD-LTE and TD-SCDMA bands which opens up use to basically every carrier in the world, this will prove to be especially useful in China." This phone is the most versatile and previously was only available in a locked version.
I'm assuming that Apple will be selling the A1586/A1524 versions as the "SIM-Free" version. Unlocked and compatible with almost every carrier in the world.
Lots of articles out there about this. I found this
one to be the clearest.
