Yup, as already said, it is a "feature" and not a bug. My wife's '04 Camry had the same feature - though it was shown in that car's owner's manual. I think it said (for that car) that if the car was remotely unlocked but not opened within 30 seconds, it would auto-relock. I think that particular book called it a safety feature, so you've got me as to exactly what is safe about it.
Maybe the designers are thinking you could accidently unlock a car as you put the fob in your pocket - in say a mall parking lot - and programs the cars to relock as said safety feature. I know at least twice a year I accidently activate the panic button on each of my various key fobs - so I guess it isn't so much of a reach to think I could also accidently unlock my 4Runner too.
I haven't messed with it much on my new 4R, but in my wife's old Camry (has since been traded), even if you remotely opened the trunk after unlocking the car - but didn't actually open a door - it would still relock the doors after 30 seconds. I do not, however, know what the exact time period is before a 4R relocks.