why must HEAD always go over the network when online?

Say, why must HEAD go over the network, but GET needn't?

Why can't HEAD just report from the cached copy according to the same
rules determining its freshness as GETs?

When online with an already freshly cached copy, it would speed up the
first step of:

$ wget --content-disposition \
'http://translatewiki.net/w/i.php?title=Special:Translate&task=export-as-po&group=core-1.14&language=zh-hant&limit=100'
--2009-03-09 14:15:05--  http://translate...
Resolving localhost... 127.0.0.1
Connecting to localhost|127.0.0.1|:8080... connected.
Proxy request sent, awaiting response... 200 OK
Length: unspecified [text/plain]
--2009-03-09 14:15:08--  http://translate...
Connecting to localhost|127.0.0.1|:8080... connected.
Proxy request sent, awaiting response... 200 OK
Length: unspecified [text/plain]
Saving to: `zh-hant_core-1.14.po'
2009-03-09 14:15:08 (42.4 MB/s) - `zh-hant_core-1.14.po' saved [437934]