BBM
It truly is in his best interest to do this and I'm sure it's been pointed out to him by his "team". You'll notice how long it took to get this "confession" out of him. I think he really had to weigh the pros and cons.
IF this was a premeditated murder planned by both of them (and I think it was, at least from the time of the argument in the lobby) and they are convicted as such, it is a devastating outcome for both of them and their unborn child. If he doesn't go along with what HM wants, he probably knows she's just crazy enough to blow the whistle on both of them (or just him) that this was a premeditated, planned murder.
If they can convince a panel of judges that this was all him in an emotional fit of rage over a racial slur and she gets off on all charges, she will be able to return to the US, collect her inheritance and start to raise their child. While also being able to send funds to TS to make his 15 years or less in Kerobokan tolerable. She can even spend lots of time in Bali with their child visiting him or relocate there temporarily to make his life easier. At least this is how it will look to TS when explained to him. Now whether she would actually hold up her end of the bargain or not would be questionable. I think she would though, for a while until she finds another guy to occupy her time. Imagine the poor martyr she'd make of herself in that situation. Torn between the love of her life and the death of her mother at his hands. Boo hoo hoo. However I'd imagine SWM's family will try to have those funds tied up for a long time and would be launching a wrongful death civil suit of their own against her, so it probably wouldn't really play out that way. But there's no need to tell TS that now is there?
MOO