I Travel for a living so I usually buy an unlocked iPhone (currently 5S).
The T-Mobile version will be locked when sold, BUT a few months after activation T-Mobile will unlock it for you. That is my understanding and that's how it worked before.
Apple isn't releasing a fully unlocked version yet simply because of the demand. In a few months they probably will. There will be a third option in the scroll down menu. Right now if you go to Apple Store on your current phone and select iPhone 6 or 6+ you'll see 2 purchase options:
1 Replace this iPhone
2 Replace another iPhone.
That's it.
if you select iPhone 5S you'll also see a third option:
3 Buy Unlocked or Contract-Free.
IF you buy the T-Mobile version they will unlock it but you'll have to spend money to open an account with them, well at least for a few months. That's my understanding. Basically supply and demand. Right now there's "more demand" but in a few months you'll see "more supply" and thus more options.
..and yes, prices don't include State taxes because they're all different in different States (just like in EU). They range from 0% (Alaska, Montana, New Hampshire, Delaware, Oregon) up to around 10% but again, different rates. (some examples: Hawaii (4.35%), Maine (5%), Virginia (5%), Wyoming (5.17%), South Dakota (5.22%), California (9.08%), Arizona (9.01%), Louisiana (8.69%), Washington (8.61%), New York (8.52%), Illinois (8.22%), Arkansas (8.10%), Alabama (8.03), etc.)
Ps. I've been told that an iPhone 6 will work in all the markets no matter what version it is.