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 decisions than expected value.
Question 5a is: $900 @ 100%, or $1000 @ 90%
Most people choose $900 @ 100%
Question 5b is: -$1000 + $100 @ 100%, or -$1000 + $1000 @ 10%
Most people choose -$1000 + $1000 @ 10%
Written this way, the false symmetry vanishes, and we see that in both cases, people are risk averse when the payoff is low, and risk-seeking when the payoff is high. Which is to say, people value life-changing sums super-linearly as compared to insignificant sums.
If I'm understanding you correctly, you're saying it's about the amounts; and I think I'd have to agree with you that for the examples to be equivalent, the amounts in 5b should be multiplied by 10. But then again, maybe that would introduce another comprehension hurdle or cognitive correction effect which would render the experiment invalid. Interesting question to ask the original researchers, although I presume that by now (after decades) they've addressed it somewhere already :)
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.
A 90% chance of losing $9000 has an expected value of -$8100.
Do you want to be guaranteed you'll lose $900?
Or do you want a 10% chance you'll lose nothing at all, with a 90% chance you'll lose another $100?
So given a choice between being (nearly) totally wiped out, or having the chance of not being wiped out, people take the chance of keeping their cash.
Makes sense to me.
If I could pull $900-$1000 from my savings with no immediate consequences, I'd be more likely to spend the $900 at 100%. But if loosing $900-$1000 means I'll have to tell my landlord I'll be late with the rent and then finding someone to borrow it from, and paying it back with interest, the extra $100 aren't significantly more crippling - it's the transaction cost of going through all this bother that's problematic - I'll take a 10% chance.
Come to think of it, I actually did something like this: Prior to moving abroad a while ago, I consulted a lawyer to make sure I did everything right to avoid double taxation. That was a taking on a 100% chance of a rather big expense to avoid an unknown chance of an even larger expense.
I also agree that the reasoning changes a lot if this is a one-time event vs. a regular occurance.
But the point is that research has show that almost all people have this bias that makes them much more risk adverse when avoiding losses than when they don't already have the money.
We can speculate a lot on the reasons, but that speculation isn't tested yet.