Only the app that created the folder on the card can write to it (or another app that belongs to the same group as the app that created it).
In Lollipop, developers will be able to write apps that can create folders that other apps can write to, and apps that can request write access to folders that other apps created. But they didn't think we'd react so badly to making the SD card part of the normal Linux filesystem when they wrote KitKat. (It's a bit of a pain to those of us who understand how to manipulate Linux - to those who don't, it's probably like trying to tear a brick wall down by slamming your face into it.)
