Again, not true, not outside the US anyway. The manufacturer makes the phone and the carriers take the ones that they think will sell, which usually constitutes "all the phones that are popular." Also, in most of the world there are no different versions of phones for each carrier. There is one version of each phone sold on every provider, that's it. The only difference between them will be a bit of branding. That's the case throughout the whole of Europe and pretty much the rest of the world outside of America. It's also standard in many countries to buy the phone and service completely separately, so in fact, the customer purchases the phone unlocked without any interference from the carrier at all.
Aside from Apple, pretty much phone manufacturer is not American (Samsung is Korean, HTC is Taiwanese, BlackBerry is Canadian, etc.) so they are not specifically designing their phones for US carriers. Hell, Apple isn't either, in fact Apple famously stood up to the carriers when they launched the first iPhone.
They do all make variations of their phones for different bands, because for some reason there's no standardised frequencies and cellular technology in the US as there is in Europe, and so that usually requires a different SoC, hence the US and international variations of high end phones like the S4. But that isn't changing the specs for the carriers, it's changing the specs out of necessity so they can use the right frequencies.
