Some applications data storage is not limited to internal storage, but they also store data on external SD card. They have access to file system and can read other application's data, instead of accessing particular folder.
This is due to limitation that most SD cards normally use the FAT file system, which does not support filesystem permissions. So, Android would have to manage a custom list of files, their permissions, and mapping of applications to files and folders. This will make very complicated and also would require a lot of storage space. If user take out the SD card from mobile device to remove pictures or other data, and the entire system could break.
Applications may require the SD card permission to store large amount of data or it has to add, modify or remove files in a file manager. But for a application specific data, storing data in SD card is not recommended because it could be removed at anytime by the user and will not be available to the application. Removable media is designed to be altered by other devices, so attempts to manage it are not feasible and prctical.