Originally Posted by Kumulani

No, nobody is forced to take the airside bus. You can just check in at T2 and walk over to TBIT, just like you'd walk from T3 today. It just means a little bit longer of a walk. The only time it would make sense to use the airside bus is if you had a arrived domestically in T2 and wanted to connect to TBIT.

I assume they did this because they wanted to consolidate check-in areas by alliance as much as possible.