You got me. I don't know much about Hinduism, and when I asked that same question, no else in the lodge knew either. I'll see if I can find out and get back to you.
Since there are a few hundred lodges in India, it appears that belief in the Hindu supreme being(s?) is sufficient. If all manifestations of the Hindu pantheon are a part of Brahma (sp?), that would seem to be the reasoning.
All one has to do to be able to join a Masonic lodge is to be of good character and profess belief in 'a creator'.Bhuddists are allowed. I have some Bhuddist brothers in my lodge.
Hindus are allowed as well, although they are not polytheistic in the way I believe you are thinking. All those deities that Hindus pay hommage to are different manifestations of a single creator.
(That's probably a gross over-oversimplification. If there are any Hindus reading this, I would appreciate some info.)