Ultimately it's up to the app developers what does and does not get backed up to iCloud. iCloud will by default back up all content for an app, but the app developers can mark certain data to exclude from backups. It would make sense for Google to exclude this data since it's already stored on Google's servers, and users only get 5 GB for iCloud backups without paying extra.
It would probably be nice if Google could remember the songs you've marked to use offline and download them all at once when you reinstall, but it sounds like they don't (for the record, neither does Apple's own iTunes Match.) there probably isn't much you can do about this.
What I would suggest is to create a playlist specifically for music you want offline. Google Play lets you download the entire contents of a playlist with one tap, so you could have an "Offline" playlist, and whenever you need to reinstall, just tap the button to download the entire playlist.