What the law says
Here are some extracts from the Competion Bureau's website :
Mass Marketing Fraud is defined as fraud committed via mass communication media using the telephone, mail, and the Internet. [...]
Under its criminal regime, the Competition Act, prohibits knowingly or recklessly making, or permitting the making of, a representation to the public, in any form what ever, that is false or misleading in a material respect.
[...] The application of the general impression test is particularly important where: which brings us to the section on the Non-disclosure of material information :
The representation is partially true and partially false, or the representation is capable of two meanings, one of which is false;
The representation is literally true but is, in fact, misleading since it fails to reveal certain essential information (Refer to Non-disclosure of material information); [...]
2.4 Non-disclosure of material information
Any information that would likely have a tendency to affect a purchasing decision should be included.
Therefore, there is a misleading omission when McDonald's omit to clearly show on the advertising material that the "game" Monopoly is in fact rigged because some stickers don't exist or have been distributed in such a low quantity that it's impossible or almost impossible to get them. Remember that to show the odds hidden somewhere deep within the rules is not enough because it contradicts the overall message of the advertising.
It's obvious when you look at this section
Do avoid fine print disclaimers. They often fail to change the general impression conveyed by an advertisement. If you do use them, make sure the overall impression created by the ad and the disclaimer is not misleading. McDonald's did the opposite : they claimed that Monopoly was a collection game (as it's supposed to be), that you could "collect-to-win" but if you read the fine print, you realize that the fine print completely contradicts the original claim that it's a collection game. It was a lie : it wasn't a collection game! It was a lottery!