[7] Cromwell and Karakatsanis JJ pointed to the possibility that cooperative federalism might work against parliamentary sovereignty and might unduly complicate situations where one level of government's policy choices affect the other's.

[16] The dissenting judges looked to the legislative history of the Firearms Act, including comments made in the House of Commons that noted the high degree of provincial involvement in the scheme.

That would upset the balance between, on the one hand, the principle of co-operative federalism—which permits a government at one level to pass laws that affect the powers of the other level—and, on the other hand, the doctrine of interjurisdictional immunity—which is inherent in the principle of federalism.The dissenting judges found that the pith and substance of the impugned provision was to hinder any provincial attempt to use the long-gun registry data, which falls primarily into the provincial property and civil rights power.

[26] The dissenting judges thus would have found section 29 of the ELRA unconstitutional, since it amounted to unilateral federal destruction of the registry data without first offering it to the provinces,[27] and would have declared it invalid.

Constitutional law professor Jean Leclair, writing for La Presse, argued that the majority's decision "weakens the principle of cooperative federalism".

[30] Sean Fine of The Globe and Mail noted that, out of the four dissenting judges, three: LeBel, Wagner and Gascon JJ, were all from Quebec, pointing to a "divide between the Conservative government and many Quebeckers".