Is sulfate free shampoo and conditioner bad for me if I have healthy hair?

I dont dye my hair or really do anything to it. I sometimes flat iron it using a heat protectant first. I also only wash one to two times a week.

We have hard water and even with a filter my hair is frizzy and not soft. I used a box dye conditioner and it made my hair so soft and shiny I want that all the time! But everywhere i read it says sulfate free is for dyed or damaged hair and that it can damage the scalp or something like that....