The ruling party kisses the ass of foreign countries, especially those in which it has vested economic interests. Hence the people of those countries are treated with kids' gloves. There is also an overt expression of nice gestures to publicly show that Singapore is not xenophobic and so please keep the foreign workers and investments coming.
The opposition focuses on human rights, because it perceives that the underlying discontent of the foreign workers are a hint at the the bigger picture of oppressed and exploited local workers. Employers get away with it because the govt and the pseudo-union allows them to do so.