Primarily always-online is due to;
- Not as easy to compromise the integrity of the community, leader boards etc. Can't do any offline tweaking of values (assuming they're doing server validation!). There are still hacks that can occur but they're less risky and typically easy to detect.
- Ensures game progress and customer data is synced across devices and the game servers
- No traditional piracy of IAPs (again, assuming they're doing server validation!)
I feel like iOS or Android does matter actually.
On iOS you can't modify/import/replace the files that you have on your iDevice since iOS8-9 unless you're jailbroken. It's way easier to modify an APK or the files that you find on your Android device. That also goes for iAP piracy which is already quite hard to do on iOS and especially here since the game requires iOS 8+.
Furthermore, you could always check for online once/when you buy (which you obviously need at that time), already ensuring that almost no-one on iOS will crack it. And anyways, even if you are using a jailbroken device with a tweak that can crack iAPs, always online won't change anything since they can't crack server-side checks and can crack the other types of iAPs while being connected.
You could also always copy the data back when you're online to the servers the way most iCloud stuff works thus rendering the excuse of "always online to ensure progress" moot.
For leaderboards I thought it was mostly Game Center that was way too easy to cheat but here, presumably, it's using Nintendo's system that could always check for discrepancies when you get back online without having to enforce an always online drm.
Although I do agree that this is probably the only explanation that sort of makes sense but still doesn't justify it for a not so F2P game.