It's not the same for curly hair. On straight hair, naturally produced oil from the scalp can more easily travel down the hair shaft, which helps hair retain moisture and protect it from drying out.
Since curlies don't brush their hair (at least while the hair is dry) and our hair shafts are bent, like NvmbrCurlss said, the oil can't travel down as easily.
THAT is why people with curly hair use conditioner, because it gives the hair moisture that we can't easily get otherwise. That's not to say that you can't wait for your natural oils on your scalp to return to a normal state after years of shampooing and wait for them to travel down your hair shaft, but you might suffer from dry, possibly brittle, hair in the meantime.
As others have said, the Curly Girl method is not a product-free method. CG instructs us to use a co-washing conditioner without silicones and/or a heavier leave-in/rinse out conditioner, which isn't completely washed out. Then, you style your hair with a silicone free gel.
Curl Style: 3b/3c
: Giovanni TTTT (my hair loves this)
: GVP CB or DevaCurl One Condition, occasionally with honey
: Biotera Styling Gel, BioSilk RHG
: AG Re:coil, CJ Beauticurls, Lube
Hair hates Aloe Vera & Avocado