The questions involving "90% chance of $1000 or 100% chance of $900" always bother me. I never understand why economists think that a rational actor would consider them equivalent; they're not , unless you are making that choice many many times. But if I'm given that chance once (which is presumably what most participants assume, since that's not a choice that comes up often in one's life), it's really then a choice…
The interesting thing is not that people are risk averse (and thus choose 100% chance of $900), the interesting thing is that people become risk seeking when it comes to losses (and thus choose 90% chance of -$1000). You're creating a bit of a straw man when it comes to behavioral economists and their view of "rational actors" - the whole field is built around the understanding that there is more to economic decision…
A = $900 lost B = $9000 lost with a 90% chance
Those are odds I wouldn't want to try. I would take the instant loss knowing that my odds are just not good enough to win if I were to choose B.