High-end Japanese hotels will often add a service charge, but that is (in my experience) completely specific to hotels. I also don't think I've ever seen a 20% service charge (10% sure).

I don't have a problem with service being itemised on a bill. If service has been poor, there is no need to pay the whole service charge. I've done this in the past in restaurants in the UK - I've reduced the service charge amount and indicated that I will be happy to speak to a manager about why.

I've never felt the need to do that in Japan however; as service is almost uniformly excellent.