First off, here's the official announcement from Sprint:
Sprint Newsroom | High-End Speed and Performance Without a Contract: Boost Mobile Strengthens 4G Lineup With the Award-Winning Samsung Galaxy S II 4G
I understand that ICS was bitter for many folks - most of them because they bought the phone and were talked up by a sales guy that it was coming soon after it rolled out. Blame the salesman for that - Sprint (corporate) was very clear that it would only come out when it met QC. Since they didn't start internal testing on it until June I'd say we got it in a good amount of time - and they dramatically cut that time on the FH13 build. They could because they tested it out using FH31, which is the same built but the Boost version.
The main goal at the moment seems to be what you mentioned - getting a rock solid ICS build. Then they'll port over JB and work on that. About that time we'll probably hit 18 months - a timeframe rumored to be the end of a support contract - and we'll hit EOL.
Can't explain about the GS since I didn't own one. Best educated guess is they learned their lesson and made it part of the support contract on new phones from that point on... or it was added by Sammy to compete further against Apple (since it
was a bone of contention with the Galaxy S versus the older iPhones.)