Originally Posted by Yaatri

That's the difference. Places that are used to catering to foreign visitors know the rules. The rest don't. What you need to get a SIM card is a copy of your passport, visa and proof of address. Your hotel/Guest house can provide your proof of address. The documents are sent to someplace for verification. If the person looking at the documents is not satisfied with the documents, has had a bad day, or is drunk, your service can be turned off. I have read about such incidents on IndiaMIke forum. Vendors even in big cities, away from touristy areas, do not know that a SIM can be given against a foreign passport. That was the problem I faced.
While I do not doubt your experience, but it's not universal. I can wager that more than 99% of vendors in the country would not give a SIM against a foreign passport. A simple solution, that's often suggested, is to have someone you know by a SIM against their ID. I am, in principle, averse to such a solution. I do not like to break any law.