It's great that you've not been busted for this, but what you're doing is technically against the rule.
I know this, because I've specifically asked this question to a couple of Jpn properties and both times was told that this only applies to double occupancy situation.

Keep in mind that the prices for guest no.3 and guest no.4 in a party of 4 are not necessarily the same as the price for guest no.2 in a party of 2. For example, one of those hotels Hilton Chatan explained to me last year that their room rates are the same for single and double occupancy, but they have significant incremental rate increases for the 3rd and 4th "adults", respectively (their adult is 6+yo). Kind of make sense since Diamond benefits are always extended to the 2nd guest anyways and the 2nd guest would not require an extra bedding, but that would not be the case once you start dealing with 3rd & 4th guests in Jpn.