Because Bungie doesn't work that way. I would think the maps would be made available on the XBL Marketplace for people who haven't purchased the game, especially once the maps become required content for matchmaking.
As for installing, the reason Halo 3 would be slower is that all the maps are still loaded off the disc regardless of if it's installed (don't ask me why, I'm just vaguely recalling what was said by Bungie). I think this is so that custom map layouts can be applied to the default one (loaded off the disc) without permanently overwriting the default layout. (which could happen if it's saved to the HDD)