It isn't. Ag-gag laws extend the scope of, or punishment for, trespass with intent to record video of agricultural operations. That is what the courts have found to violate free speech and equal protection.

It is as if states enacted laws against graffiti bearing a particular political or religious message, with either a greater punishment, or broader definition of "graffiti", than existing graffiti laws. Yes, graffiti is already illegal; yet these laws clearly violate free speech and equal protection, since the punishment is different based on what message you are trying to convey with the graffiti.