Depends on the restaurant. I have it on good authority that casual wait staff at a certain Western-owned international hotel chain don't get a share of the service charge.
I've also been told that foreign managers, who are already on decent packages, get a share of the service charge.
No wonder the taxi and tuk tuk drivers try to "rip off" the tourists, they're just following the example set by the purer-than-the-driven-snow foreign companies ????