The problem is not that they intervened on behalf of the Thai woman, it's that a gang beat the cr.p out of intoxicated, more or less helpless, admittedly, idiots, and thoroughly relished the experience. One or two punches would have been sufficient, anything beyond that had very little to do with the initial slap of the girl, much more about sadistic, possibly xenophobic enjoyment of beating a farang unconscious.
The slap was just a needed pretext. There is a lot of pent up aggression in a society that on the surface is extremely polite.