Arun Jaitley (the Minister of Finance) stated: "Through multiple notifications, the two parties (the government and the RBI) have notified the public over and over again that bitcoins will not be considered legal or legal tender in India"
The Government of India has formed a committee to regulate cryptocurrency currencies.
So,
"Legal tender versus being illegal"
Being "illegal" and not being considered "legal" are two completely different things. Cows are not "legal tender" in India, but are used for trade and barter. The fact that a government does not consider it "legal tender" does not mean that it is "illegal" to use or possess. But they try. India and China tried to ban gold at 0.01% a few years ago, and even the United States banned gold at 0.01% in the 1930s, all it did was force people to accumulate. The best they can do is try to "scare" people and accumulate the underlying assets they have made in 0.01% gold and probably also in Bitcoin.