Coming from a T-Mobile US perspective, keep in mind that those 640's were wiped off the fact of the earth very quickly. I remember looking back in the fall and unless I wanted to risk an eBay purchase, I could not find that 640 anywhere.
Now imagine you're a TMo user who doesn't know anything about AT&T phone unlocking / debranding, and want a decent BAND 12 Windows phone. Your only option, assuming Tmo eventually offers it, is the 650. So I don't think it's so much MS treating us like idiots, it's more that MS doesn't necessarily want users hacking those ridiculously cheap clearance 640's.
EDIT: Wasn't aware of the Alcatel (BAND 12 Tmo Windows phone). Still, no 640 anymore, hasn't been for many months now.