Allah (swt) presented to us a picture of the physical abuse in the matter of giving less in scale and weight and how the high rank people control the weak in their food and living morsels. Here Allah (swt) is talking about the moral and psychological abuse:

Making fun, mockery, irony, taunting, superiority with falsehood and boasting in the face of the truth, Allah (swt) clarifies these expressions so that we know that their greatest crime is the betraying of Allah (swt).

(Used to) refers to past, the time of it has ended, while the word (to laugh) in Arabic came in the present tense, as if the verb is happening now. Allah (swt) did not say (those who committed crimes laughed at those who believed), to visualize the picture of this case and how it harmed the soul of the believers.