Laws are not research papers, they don't need to cite sources.

The fact that they do not need to is true, but the typical law rests on a body of data and some of that data will be supplied with the bill depending on what the material is about.

This is because most laws are created with specific goals and purposes in mind, they're not born finished and ready to be voted on derived from first principles, they usually exist to address something quite specific.

I never said they "can't", I said they shouldn't.

Also, there's nothing fundamentally wrong with citing something as an example in the argumentation around a bill enacting a law (unless it's by way of targeting them with polemic), but there is something fundamentally wrong with putting that in the legal code itself.