I worked at Windows Phone leading up to the WP7 launch all the way through the last version. The carriers had two things that every device maker, including Apple (initially) needed
1) distribution - Americans went to the carrier store and not Best Buy for their new phone
2) financing infrastructure- they could finance, subsidize and generally hide the up front price of these devices. The market expected a $0-200 phone with contract.
So, you needed AT&T and their contract prices already assumed they were subsidizing the phone so it was no point selling unlocked, full price.
Samsung was paying mobile store reps (at AT&T, TMO etc) $50-100 PER PHONE to push Samsung ($9B marketing budget).
1) distribution - Americans went to the carrier store and not Best Buy for their new phone
2) financing infrastructure- they could finance, subsidize and generally hide the up front price of these devices. The market expected a $0-200 phone with contract.
So, you needed AT&T and their contract prices already assumed they were subsidizing the phone so it was no point selling unlocked, full price.
Samsung was paying mobile store reps (at AT&T, TMO etc) $50-100 PER PHONE to push Samsung ($9B marketing budget).