In 2002 mass tourism was very much there, and so were the touts offering anything and everything.
Local mafia rules mean that the 20B needs to be collected on the island, and not added to the ferry ticket.
Bear in mind that electricity on the island is more than the mainland, and getting (and keeping cold) drinks such as water costs more than on the mainland. Therefore, prices are higher.
This is a sad but unavoidable case of somewhere getting too popular for its own good, and it happens all around the world.
I lived on Phi Phi during the filming of The Beach movie, and ever since then its popularity has sky rocketed.