It's because Honda Europe pre-defined and pre-ordered some months ago the specific EU trims from Japan, in order to deliver them quickly in the various countries.
So any optional is in fact added by the local dealer, hence you have to pay not only the single accessory item but also the hourly rates of the dealer...
Wheels are no exception here, they will not sell it as a trim upgrade but as an aftermarket second set. This means they're pricing it as single wheel, like other car manufacturers.
In my case this is good, because i wanted a second wheel set to be used with winter tyres.
Just make an example, I had a similar situation with my main car (Audi Q5 55TFSIe). The upgrade wheels, that in configurator costed 1.000 EUR (obviously removing the OEM wheels, like as always, so you're paying only the price delta) came up to cost 6.000 EUR if bought aftermarket. Luckily it wasn't me paying that...