Reputable polls such as Angus-Reid, Nanos, and many others, base their results on a sampling of people contacted over a specific period of time of about 24 hours to provide their reaction of a specific issue on a sliding scale- bad/good, agree/disagree.   Typically, it is a series of about 10 questions relating to a specific issue that gauges the knowledge base of the person about the subject at a particular moment in time.
Khadr was a national social issue and the polling results collected reflected sentiment felt across the nation.  71% of Canadians outright disagreed with the decison of the government.  
The interesting part was that subsequent polls did not immediately translate into support for the political opposition.  This reflects that Canadians strongly opposed the government's decision but at this stage, it is not an issue that has undermined support for it.  The biggest issue to date impacting popular support for the government has been the  ongoing cash for access scandal.