Actually, on some rubber dome keyboards, the keys aren't detachable. And, on scissor boards, it's usually EXTREMELY DIFFICULT to remove the keys without breaking them.
I really should be more careful with my wording, though... I'll try to standardize on this, in the future:
Cap = top part of a key, removable on many Model Ms separately from the rest of the key
Stem = bottom part of a key, engages the keyswitch, separate from the cap on many Model Ms
Key = cap and stem assembly
So, the Model M13 has removable keys, but they're one-piece.