Actually it is similar practice in France. The difference primarily is due to taxes - local taxes in many cities are based on the number of guests - off the top of my head, Italy. France and Spain all have these tax issues - much easier to detect when booking with points as local taxes are charged at the hotel level and not covered by the programs. When I used the points from spousal account and he was not coming, I put in 2 guests and added the additional guest name (me) to the reservation - this avoid tons of annoyance at check in. When booking out from my own account and I am the only one to stay I put in 1 guest.

The only country I know that indeed charge more on room rate based on Single or Double Occupancy is Japan.