I am not sure why pick on AI. They offer OLCI for many city pairs including MAA-SIN but just not for SIN-MAA. Even Jet and KF do not offer online checkin for every city pair they serve.

This is not too uncommon. I fly AC a lot from the US and they do not offer online checkin for plenty of city pairs in the US they connect to from Toronto / Ottawa.

Thats just the way it is not sure picking on them makes it any better.