I know that is Nikon's policy in the USA and is likely the case in Canada as well. If batteries are included in Nikon Canada's policy any Canadian Nikon Authorized Dealer should be able to confirm.
I was thinking the same thing regarding the higher import duties or some law or special tax on Li-ion batteries. Though if that was the case, the additonal cost should be reflected in the Manufactures List Prices across the board. However the EN-EL4a and the EN-EL18 both have a lower list price in Canada. Good Luck and Enjoy your Nikons!