Yes, it is by design. Locking screen just there to prevent you from accidentally moving an element, and does not prevent paste or delete (it is not meant for preventing changes to the screen).
When you select an element and press the arrow keys, you usually want to move the element (using the keyboard is more for moving the element 1 or few pixels to a specific location, since it is more accurate / allows moving by only 1 pixel and only 1 direction).
The Lock of specific element was added later (in version 1.18) along with the improved HMI (speed, memory and features), which later allowed adding the Layers support (I think version 1.19)