Easier inventory management for stores like Best Buy. Instead of having to carry SKUs for the T-Mobile, AT&T, Verizon, etc. carrier-locked variant of each phone (which, I'll admit results in some combinatorial explosion), they can carry 1 SKU for each truly different hardware configuration. Then it magically "becomes" an AT&T locked phone when they insert the SIM in the store.

If you buy it subsidized or through a carrier payment plan, this works as intended (and they'll refuse to let you have the phone before the SIM is inserted). If you meant to buy an unlocked phone, you've been given an annoying lesson to buy directly from the manufacturer.