just got off CS chat. I begged enough about getting the $10 (in addition to my $15), and while they couldnt give me the deal, they made it clear they would go back and fix the error with everyone else) (/sarcasm)
I wouldnt be at all surprise if Amazon was doing the double credit intentionally. Those who pre-order early are obviously enthusiastic about games, and probably shop around. There cant be that many of the double credits amongst casual gamers, and they probably have a set number they give before they bump the credit to the "real" one. Locking you in to a $10 VG credit ensures they'll spend $ on things that typically cost between $30-60, instead of Gamestop, BB, etc.
The $15 Amazon credit is a bit more of a hit (on their part), since there are many things (i often buy books for school, that i was gonna buy anyway, just nice to get them for free or cheap with the credit)
so?
locked in enthusiastic gamers to a game purchase (now) plus another game purchase in the future, plus another amazon purchase? Check
locked in moderate gamers to a purchase (now) plus one in the future? check.
my long winded point? you are not tricking them with a secret deal. i think they know exactly what they are doing.