Actually, Siam's maritime bonus IS working, but definitely not working as it should. It's as weird as it gets, I must say.
I've tested this a bit (which was a pain: my PC is long overdue for an upgrade, so load/turn times are sickening), and there are still a dozen things I'd like to test but I just haven't got the time right now. Maybe you guys could finish with the investigation.
Okay, so here's what I found out:
1) If you befriend a city-state 1 stage at a time (do something to become friends, then something to become allies), the +50% is never added. You get +2/+1 for step 1 and +2/+1 for step 2, for a total of +4/+2 - same as any other civ.
2)However, if you do something to go from neutral to allies at once, you get +6/+3 - what you actually should get, taking +50% into account. If, from allies, you declare on the city state, you correctly lose the whole +6/+3 bonus.
3)Now this is where it gets interesting. When you drop from allies to friends status, you only lose +2/+1, leaving you with a +4/+2 bonus (double of what any other civ would be getting at this point). If you go up to allies again at this point, you'll be getting a total of +6/+3 again.
4)When you drop from friends to neutral status, you again lose only +2/+1, leaving you with a +2/+1 bonus, even though you are neutral! At this point you could even go to war with the city-state, the bonus still stays. It kinda becomes an integral part of your civ.
5)If at this point, being neutral, you go to allies in 1 payment, you get +6/+3 again. Which of course stacks with you brand new +2/+1 civ bonus =) for a total of +8/+4.
I suspect you could rinse and repeat for infinite food (didn't test this: too troublesome to click through 30 more turns with random events messing things up).
6)I don't yet know how all this stuff works afrer Renaissance, when the "normal" maritime bonus increases to +6/+3 (I think).
7) Finally, the city-state info pop-up doesn't take the Siamese bonus into account, it just shows what it shows for everyone else. That is a separate display bug.