From what I've read (and I have to admit complete ignorance of the inner workings of these things myself), the OEMs offer the devices to the carriers. The carriers can bid on an exclusive or just to carry them non-exclusively. I suppose that an OEM could withhold a device from a carrier but given that WP7 has such a small footprint in the market, I doubt they would withhold it from a carrier.
There would be differences of who has the power in the deal. For instance, when Apple first came out with the iPhone, they were in the position to make some big demands on the potential carriers and only AT&T was willing to agree to them. The small market share would give the carriers the upper hand and I would think that the sheer number of various Android devices would weaken the OEMs position as well.
Because they want you locked into a 2 year contract, and they don't want you taking your phone to another carrier. One of the things that Nokia had to do to get back into the U.S. market was to stop selling unbranded/unlock phones in the U.S..
Umm yeah..*** is prety much dead on.... carriers prety much decide if they get phones. Ovbiously OEMS have the final say , HTC ,samsung w/e could refuse a phone contract to a carrier for not meeting their needs or what not.... ( like AT&T can say we want exlusive acces tot he HD7 ,, HTC could say* NO. )
and just to add to what everyone said.
CDMA chips cost MORE than GSM/hspa Chips to make...
Back in the day Bell dint have ANY HSPA phones ( only CDMA ) and NOKIA stopped making phones for bell cause it was costing them an extra 100$ to make each phone for CDMA use. ( so they made a lot less profit then GSM )
now that was like 3-4 years ago . im sure cost went down, but still expensive. + CDMA chips tend to be a little bigger , maybe harder to fit in our new slim phones.