Well, its got the "start on boot" permission, along with a ton of other apps that do too.
I suspect its really because it has/hosts more than one service. Since its pretty much a core app for Google, it (or its services) is/are probably referenced (but not always needed) by multiple other apps/processes.
I downloaded the most recent update for Maps a couple of hours ago, and I've noticed that Maps has not used any CPU time, so I think it won't use any significant system resources until its actually used.
Cheers!