dont really understand why the japanese are doing it, or why it started?
Aboriginal and later Japanese people caught whales from shore since prehistoric times, in small numbers, much like aboriginal people in Russia & Canada do today.
Whaling was practiced in some areas, and in other areas was not, particularly since people in some regions thought the whale sacred.
Taiji in Southern Japan was a whaling town, and in the late 1500s whalers there developed improved techniques for teams of men to hunt using harpoons from open rowboats, much like the the whalers from America and Europe would learn to do at a later time. Whaling went from being a subsistence activity to an industry, and soon spread to other parts of Japan.
In the mid 1600s a man named Kakuemon Yoriharu invented a brand new technique for catching whales inshore where spotters on shore-based towers would look out for whales, then a group of boats would go out past the whales and start using noisemakers in the water to panic the animals, and the fleet would shape their line to drive the whale, or a few whales, toward shore. Just offshore another group of boats would be waiting with nets, which they would deploy just at the right time to entangle the whales during their rush ahead of the noisemaking boats. The whales were then easy to finish off, with much less effort and risk on the part of the whalers. The whales were then towed to shore to be flensed.
The industry became so efficient that conflicts over coastal pollution began to develop between the whalers and other users, especially fishermen, since enough blood & offal were being released along the coastlines to cause horrific smells and water fouling, as well as eutrophic destruction of local ecosystems.
Whaling continued apace in towns such as Taiji, while local opposition shut down whaling operations in some other towns, but once modern factory ship whaling with bow-mounted harpoon cannon was introduced to Japan in the 1890's the writing was on the wall. By the 1930s whales had almost been wiped out from the Sea of Japan and the Japanese Pacific coast, and shore-based whaling wasn't feasible any more.
Local whalers in Taiji knew that their techniques worked for all cetaceans, and whale stock depletion forced them to switch over to smaller animals such as dolphins, pilot whales and the occasional false killer whale- then after World War II, a serendipitous development put them in a unique position to make better money than their whaling ancestors ever had. The rise of aquatic theme parks with captive dolphins meant there was a burgeoning demand for dolphins captured without injury-causing techniques. Buyers were willing to pay what must have seemed like staggering amounts of money for the live dolphins- a good specimen can bring in over $100,000, and the whalers in Taiji and a few other whaling towns knew how to do it- you use your sound technique to drive the animals into a cove and then pen them in with nets after they're inside. Then the buyers come & pick out the animals they want and have them slung up for transport. The rest used to be then slaughtered.
Starting with the expose of dolphin hunting at Futo in 1999, then several subsequent exposes in Taiji, the latest being The Cove documentary, the Japanese media have been publishing stories on the heavy metal contamination in dolphin meat, and dolphin meat has become virtually unsaleable in Japan now. During the 2009 dolphin drive season in Taiji, the whalers let the dolphins that weren't selected by live animal buyers go back out to sea, since it wasn't worth slaughtering them.