[AChimp]

Canadian law requires a skill testing question in order to give things away for free in a contest. If there was no skill-testing question, then it would be classified as a game of chance (ie. gambling) and subject to a whole bunch of other laws.

They put simple math problems on to avoid the paperwork.

EDIT: Some contests ask you the question when you go to pick up your prize rather than putting it on the entry form.