Well I guess each verson of the firmware handles the ejection of the card differently, for example in the earlier firmwares, pictochat freezes, but in the newer versons the colour changes.
If you're wondering why pictochat crashes when you take the cart out, I'm not sure, but the DS does freeze if you're in any aspect of the DS interface. It probably has something to do with the firmware checking that there a card in the drive. So maybe it's like a computer in like if you plug a usb into the computer, start to transfer the file and take it out, it freaks out. But then again, the DS shouldn't be accessing the game card while pictochat is running, as it doesn't need to, and it also works without any card in the slot.
It's probably some random unintentional thing nintendo put in?