Earlier quoted context omitted.
A currency is useful only if you can buy products and services with it. In India, you buy things in Rupees. Tou can't buy Rupees with dollars without an ID at which point the government knows anyway it's probably just simpler to pay your taxes
There is Bitcoin though
Also remember that Bitcoin isn't really anonymous and it's fully traceable. So the police merely has to do some rubber hose type de-anonymization of which Bitcoin address was the payment made to you in and from there on, all your transactions can be back-traced.