Simple. There's no notice that pops up saying "Warning, we're not going to ask for your password for another 15 minutes. Don't give this device to anyone else who might buy something with it."
Because we need warnings about everything these days. We need pop ups even less. Learn how to use the software and be accountable.
People who don't spend a lot of time buying apps probably have never noticed or figured out why and when they're asked for their password.
I don't spend a lot of time buying apps, but yet I know how why and when I will be asked for a PW
It has nothing to do with poor parenting. As I pointed out above, lots of big brothers and sisters loan their phones to little brothers, nieces, etc all the time... and have the same problem.
So, maybe this is about education more than parenting. If big brothers, et al. are loaning their phones immediately after making a purchase, maybe they need to realize the implications that their actions may have.
OK, I see that you are taking the "poor end user didn't know that could happen and is now stuck with a $400 bill" approach. My approach is that said user should learn the finer points of online purchasing and realize that, just like if they left their amazon account open on the screen, someone else could potentially make (much bigger) purchases.
I fail to see how this is any different than my kid picking up the phone at home and calling long distance and racking up a huge bill. I wouldn't blame the phone company, right? What about if my kid orders pay-per-view on my TV? Do I call DirectTV to complain?
Apple has gone through enough authorization and validation measures that we do not need any more. What we need is people to be accountable for their actions.