But that's where the benefit stops, the same benefit is also a disadvantage because petrolatums and mineral oils leave a heavy film over hair making any following moisturising efforts hard or even completely pointless.
Petrolatums and mineral oils have no moisturising ability at all!
If your hair is looking sleek and shiny after you use them, it's nothing to do with moisture, these chemicals just happen to be greasy-textured so that glow is purely cosmetic.
If you have ever relaxed your hair you will remember vaseline being put round the edges of your hair and on your ears to prevent burns. That is how strong a barrier petrolatum is. Vaseline is pure petrolatum.
Because it is cheap!
Should I immediately stop buying anything with petrolatum?
Not entirely. I recently learnt from the Science of Black Hair that the higher an ingredient is listed on packaging, the more of it there is inside the product. So if mineral oil and petrolatum are listed quite low down, there isn't much.
Personally, I don't think petrolatums and mineral oils are entirely bad. Nothing keeps my lips crack-free like vaseline and when I was a kid good old cheap vaseline kept my skin looking shiny. BUT there is one difference between skin and hair: skin is constantly being rubbed by clothes, people etc so the petrolatum rubs off. Your hair is not being constantly rubbed, if it was you would probably have mega-high breakage rates!
Ref: Science of Black Hair, page 83