Further looking into it, it seems this is the Broadcast Consortium's stance:
Following the 1988 federal election, a prosecution was instituted by the Green Party against CBC, CTV and Global, claiming that these broadcasters had breached the Television Broadcasting Regulations, 1987, because they had not included the Green Party in a leaders' debate during a federal general election and had failed to provide the Party with some accommodating time. In R.v. Canadian Broadcasting Corporation et al., [1993]51 C.P.R.(3d), the Ontario Court of Appeal held that debates were not of a partisan political character. The Court believed that while the participants in a debate may very well be partisan, the program itself, because it presented more than one view, was not. The Court therefore ruled that debates were not covered by the relevant section of the regulations. The Supreme Court of Canada refused to grant leave to appeal.In view of this judgment, the Commission will no longer require that so-called "debates" programs feature all rival parties or candidates in one or more programs. The Commission considers that licensees will have satisfied the balance requirement of the Broadcasting Act if reasonable steps are taken to ensure that their audiences are informed of the main issues and of the positions of all candidates and registered parties on those issues through their public affairs programs generally. The Commission still believes that news coverage should generally be left to the editorial judgment of the broadcast licensee.
Basically, as long as they've given some reasonable coverage elsewhere of smaller parties, they do not feel that there is an obligation for these parties to be present in the debate.
