Rapists in India rape for the same reason that rapists in other parts of the world rape. Take South Africa, for example, I believe it is the rape capital of the world, I remember reading in wikipedia that 1 in 3 women there have raped by someone before

http://en.wikipedia.org/wiki/Crime_in_South_Africa
India has an absolutely unique culture of its own about treating women. It may be looked upon as gender inequality and hipocrisy. For eg its ok if a man smokes in India but if a woman smokes it surely will attract lots of attraction and comments. In Singapore, I see more women smoking than menSouth Africa no longer has the highest rapes per capita, but rape and sexual violence is still a problem.[15] The incidence of rape has led to the country being referred to as the "rape capital of the world".[16] One in three of the 4,000 women questioned by the Community of Information, Empowerment and Transparency said they had been raped in the past year.[17] More than 25 per cent of South African men questioned in a survey published by the Medical Research Council (MRC) in June 2009 admitted to rape; of those, nearly half said they had raped more than one person.[18][19] Three out of four of those who had admitted rape indicated that they had attacked for the first time during their teens.[18] South Africa has amongst the highest incidences of child and baby rape in the world.[20] [21]

Now in big cities like Bangalore and Mumbai though there a lot of westernization and its common to see young girls smoking especially those working in Call Centres but 10 years back, that wasn't the case.