Tour members may wish to take advantage of being on the spot and extend the stopover in Singapore for a few days. In this case, the free hotel room does not apply since this is a voluntary stopover.
The Singapore stopover is a popular feature of flying with Singapore Airlines, and there are a number of cost advantages offered. See the Singapore Airlines web page that describes stopover offerings. The "Rates" link in the column on the left lists hotels.
If you decide to do a Singapore stopover, tell the Adventures Abroad agent when you are booking international air.