They have the results of the 2005 referendum to back them up too. I wasn't here at the time- was that general extended trade or Sunday trade only? Is there a significant difference in public opinion between the two? Of course, they have since supported an extended trade policy while they were still in govenment, which dissapeared in the name of partisan politics this time around.
To answer the original question though, surely no-one at Labor HQ thought asking people in a shopping centre in the middle of the day whether they needed extended hours constituted robust research? Perhaps I'm being too forgiving in assuming they were just being crafty rather than stupid.