There is nothing disrespectful about it. 1 million users is an incredible milestone, and we are celebrating that with a prize to that millionth customer.
It IS an incredible milestone. But really, you wouldn't have a millionth customer without the 999'999 "old" customers. I think it's disrespectful, because it makes me feel as if I'm a less important as an existing customer than the new bloke registering and buying something.
As I mentioned before, a $100 gift certificate for customer one million, and then some kind of draw for the big prize would seem alot more right. Something like "Everyone who bought something within 48 hours counting back from the time of the millionth purchase has a shot at the grand prize".. It would give existing customers a chance to win the really nice price announced (and likely increase sales too as a bonus).
It's my opinion on the matter. Even if you hade done it the way I suggest here, I'd still have no chance to win since I can't afford buying anything at the moment. I would still feel better though. I always get disappointed by promotions like this, because someone wins a grand prize because tons of other people made something possible.