Because the electro-magnetic field polarization is preserved in many cases (specular). That doesn't mean there isn't a plasmon generated in between or that the sign of the electric field is maintained. What is interesting is that in the case of metalic (from low to high index of refraction) there is an inversion of the electric field. This does not happen at the reflection from a high index to low index (e.g. under water looking up to see total internal reflection the polarization remains the same). You can see this same effect bouncing water waves off of a concrete wall.