Because if your route queries (and, to a lesser extent, map tile downloads) don't go through Google's cloud, they don't get to see what you're doing, and they don't get to monetize it.

Precaching the map tiles for your query (that still goes through Google) would be a useful half-way house, though...

Most people would complain like hell if their all-in-one GPS uploaded their searches and driving patterns to Google, but because Google Maps is _all_ in the "cloud", they don't mind. Go figure.