That message is produced by the usb-storage kernel module. It just means that the device is writable. Some usb storage devices have a hardware switch to make them read-only, but most of the cheaper ones to not really have such mechanism, in that case the message will still be displayed. This has nothing to do with the permissions of the device, or with the fact that you mounted it read/write or read-only. That message is casted before the device is mounted, when the hardware is detected and assigned to a driver at kernel level.
Why are you asking this? Is it maybe that your device has a hardware switch for read-only but it's not working?