Since hair is dead, what use can the hair make of vitamins in the shampoo?
The reason is to add perceived value. Add $0.01 worth of vitamins and you increase the selling price by $0.50.
In other words, its BULLSHIT! 
No, bullshit is real-poo 
Consumer Reports studied this for an article in their January 2004 issue. It’s not online, but a summary can found here.
Some shampoos were better than others, though:
http://couponing.about.com/cs/beautyproducts/tp/top10shamp.htm
Along with plant extracts, essential oils, and all the other nonsense they advertise the hell out of. That stuff won’t do the hair any good - as you said, it’s dead tissue. And large molecules like these can’t penetrate the skin, so they won’t ever even come close to living cells. Further, they’re usually added in such miniscule quantities that they couldn’t possibly help anyway.
It’s advertising hype, BS, pure and simple!