This issue was mentioned before at the cutwaste website.
http://app.mof.gov.sg/cutwaste/suggestionview.asp?id=6855
This is LTA's reply...
"We are glad to inform you that we are already doing so for our new
bus shelters. The bus shelters were upgraded under a new approach
whereby the appointed contractor would build, maintain and operate
the bus stops for a period of time in exchange for the rights to
derive income from the sale of advertisement space at the bus
stops. This approach has enabled LTA to upgrade more than 3,000
bus-stops to enhance commuters' comfort and convenience without
incurring additional costs as the contractor bears the cost of
building and maintenance of the bus stops. "
Hope this clarifies.