[Cryptography] Why is a short HMAC key zero-padded instead of hashed?

[Ron Garret]

The HMAC spec says that the key should be hashed if it is longer than the block size but zero-padded to the right if it is shorter.  Why?  Why not hash a short key?  That would simplify the code without (AFAICT) compromising security in any way.

rg