[pcl]

the ability for Google Maps to pre-download map tiles [is] available to Google on iOS

Google Now will automatically pre-download the map vector data before your trip, without you asking it to. For Google to do the equivalent on iOS, they'd need to be able to perform periodic background jobs, which isn't something that iOS supports currently.

[Xuzz]

It wouldn't be perfect, but the Google Now app could recognize that you're nearing an airport — something iOS does support, with background location updates — and then download the map data when it gets that notification. iOS apps can also share data between apps from the same vendor (via the Keychain, pasteboards, or various other mechanisms), so there wouldn't be an issue with passing that data over to the Google Maps app for offline viewing.