When travelling through India (and to a much, much, much lesser extent in Thailand), you meet a lot of people who smuggle drugs.
The facts are simple:
There is lots of marijuana grown in the north of the country (esp. the Himchal region).
There are backpackers all over India, some rich, some poor.
Many backpackers want to smoke cannabis, and are willing to pay a good price for it.
Many backpackers are broke, and want to keep travelling.
This is perhaps an over-simplification, but in general, there are two types of person smuggling ganja:
1. Someone who is doing it for the money
2. Someone who is doing it because they are going somewhere where it will not be readily (and affordably) available.
I've seen lots of Israelis sneak a kg or two out of the Manali region by hiding it inside their Enfield Motorbike, I've seen people cutting up their shoes to hide it under the in-sole, I've heard a number of first hand accounts from women who've hidden it in areas that Indian policemen will never ever search (even if they'd love to try), and just recently, I met an Italian man on the Andamans who bragged that his bottom currently resembled a cauliflower, after having ingested and pooped out a large quantity of plastic-wrapped hashish.
Almost everyone you meet has their own patented and foolproof method of smuggling drugs. You hear all kinds of stories involving vasoline, shampoo, peanut butter, coffee grounds, and ghee (clarifed butter).
In Varanasi, I met up with some friends of mine who I had initially met at Paradise beach in Gokarna. A week after I left, the police held a semi-raid on the beach, and one of the Israelis was locked up. It took a few weeks and multiple kickbacks (to the extreme that his friends had to buy a computer-printer for the Police Station, so they could print out his release form) before he was released. And he was just caught for smoking, not for smuggling.
The logic behind it is simple I suppose. People believe their chance of getting caught is very low, and that if they get caught, they'll be able to bribe their way out of it. This is often the case (although, when they do meet the rare variety of honest Indian policeman, they're in for a very very nasty shock).
The economics of it are simple:
Step 1. Buy 1 KG of charas in the north of India at 10-20 rupees per gramme.
Step 2. Smuggle it to Goa.
Step 3. Sell it 100 rupees per gram to other backpackers.
Step 4. Profit (approx 80,000 rupees, or $1800)
Given that you can live in India for a good 4-5 months on that kind of money, it's easy to see the temptation for someone who has run out of money and faces the threat of a return home.
However, there is no such thing as a free lunch... and this kind of business is so dangerous that the Indian drug dealers don't even smuggle it themselves - they hire Nepali mules to bring it down from the mountains... They're not stupid, after all...
But the backpackers are. For a mere $2500 (when you include the cost of a ticket home), they're willing to risk spending a long time in an Indian prison.. when they could probably work as a waiter back home, and make that in a month....
Interesting enough, you hear very very few stories here in Thailand about backpackers smuggling drugs...or at least, people don't brag about it and offer to sell to you the way they do in India. Perhaps it has something to do with Thailand's much publicized extreme willingness to give foreigners the death penalty for that kind of thing.