In different parts of SriLanka, different "laws" exist.
One such example is in Arugam Bay - famous for surfing.
The locals have created a "law," even enforced by some local police (with a little monetary encouragement I'm sure) that says you are not allowed to have more than 2 surfboards on the roof of any tuk-tuk, even though, including the driver there is room for 4 passengers.
The local tuk-tuk drivers explicitly forbid us from putting 3 boards on and even went as far as to block us from leaving the guesthouse whilst calling the police on their mobile phones when we objected to it being ridiculous. They then openly admitted it was so they could make more money. In other places, Midigama for example, we saw tuk-tuks with 4 or even more boards on their roofs on a daily basis.
But not in Arugam bay.
You see, the best surf spots are about a 1000 rupee tuk-tuk ride away. And when you can also rent and drive your own tuk-tuk for the same price, and take 3 friends and their boards, that means that they are losing money in their minds. Instead, even if you have your own tuk-tuk, for 4 people and their boards to go surfing you now, by "law" need 2 tuk-tuks.
The local tuk-tuk drivers literally got together and decided that the new local "law" was 2 boards maximum.
Also, in Hikkaduwa for example, you see many tourists driving scooters with a surfboard side rack. These, in Arugam Bay, are also "illegal." I don't know how they would stop you from entering or leaving the area, but we were told they were not allowed there.
Just a friendly word of warning for those planning a tuk-tuk trip with surfboards.