As someone who works in e-commerce related to phones, I think a big part of the answer is that the US is not a big market when it comes to selling phones without a contract.
In the EU it's common practice to buy without a contract, whereas this is very rare in the US.
In other words, carriers have way more power in the US.
> I think a big part of the answer is that the US is not a big market when it comes to selling phones without a contract.
If that was true, then Wal-Mart wouldn't bother carrying every pre-paid SIM under the sun in the same section of the store that sells shitbox Motorolas, nor would Amazon bother carrying the same exact Motorola shitboxen - all unlocked, mind you.
In the EU it's common practice to buy without a contract, whereas this is very rare in the US.
In other words, carriers have way more power in the US.