Fin whales are certainly endangered. There are different types of minke whales, but the relatively abundant population of the most common ones is only due to the whaling restrictions being observed by most countries. In the 1970s, Japan, in addition to its won huge scale whaling, was also secretly buying whale meat from the Soviets, who were lying about their catch numbers in their reports to the IWC, so they could rake in more cash to try to save their sputtering Marxist pseudo-paradise. Last year, the Japanese also announced their intention to hunt humpback whales in the southern sanctuary area, though I think they eventually backed off from that under extreme pressure from the Australian government and IWC. The Japanese behavior with regard to whaling has simply been dishonorable.
A lot of money changes hands in Japan over whale meat.
I'd say the motivation is plain avarice.
If you wanted to stop this market, just find a few whales infected with Mad Cow disease.