The problem in english is that “for” can have multiple meanings in this case. It can mean the product is just meant to be used on item X or it can mean it intended to make the product more like X.
In this case, I’m confused because some people with curly hair hate it and want it straighter, so “for curly” hair might make it straightens or at least not make it curlier, while people with straight hair who DO want it curly would want a intentional product “for curly hair” to make their hair curly.
Which is it?
If you want to think about it grammatically, it would be “for curling hair” or “for straightening hair” if the function of the product was to make your hair more curly or more straight.