Immorality is extremely nuanced and heavily influenced by society. Eg. homosexulity is illegal in some south Asian countries such as Indonesia, but is not seen as immoral in most parts of the world.

Conflating the two - or preferring the term "immoral" over "illegal" inherently lends itself to allowing people to rationalise illegal activities - such as child pornography as only being "disapproved of by society" (morality being a fairly abstract concept) rather than an act which violates the laws of the land and is officially and formally condemned by the government.