Generally speaking, you have the option to decline CDW in Jpn. However, you're trying to book through Holiday Cars. I'm not familiar with them and I'm not sure if they're 3rd party OTA or aggregator. But if you're booking with them, then perhaps that's their policy... that you can't decline CDW. You'd have to look into that. It's like when I travel for work and rent car in the US and I'm mandated to book cars through my employer's corporate travel agency. The corporate travel booking, in my case, will always include CDW and I have no ability to waive that.

The only other potential situation that I can think of where you can't decline CDW in Jpn is if you're booking directly with the agency but you happened to select a plan that includes CDW.