If Amazon would be a physical store in, say, the UK, they would welcome customers from India.
A web store can do the same thing, but for any (potential) customer, it doesn't matter at all where they come from, but where they are at the time of the purchase.
I don't think Amazon is aiming at welcoming Indians outside India, because they are very unlikely to be wanting to pay in rupees.
At the same time, if I were living in India, I would be a customer from the Netherlands, but in India, and I would be interested in the possibility to pay in rupees.
In short, they use in because they are targeting people in India, not Indian nationals.