I think there are two reasons for that. If brudder works as a tua sa at a certain club, his SS members will support by patronising the joint to show support. Face very important. Biz reason to employ these chaps. More tua sa from various groups also means if trouble comes, better chance of suppressing the incident from escalating coz S'pore is a small place. Someone will know someone's someone! If I run a joint, I would do the same. Yet to know a joint that employ all from the same SS.
Yes tycoon tua sa is from si tong, but under HLS territory. once in a while, si tong people will come tycoon and patron, so long as no trouble, business as usual.