Some forest are meant to burn yes, the problem is that with climate change forest fire now burn further north and for much longer than before.
Drought conditions and the heat traps that are cities prevent proper rainfall which stops them and changes we've made to the landscape can prevent burn outs which also make fires last longer.
And more importantly while forest fires are natural it doesn't mean it's good, we had at least one fire related extinction event, and even considerably smaller scale fires can be disastrous to humans so while it's true that to some extent the policy is let it burn in this case it's not longer really the case.
And while I would also agree that Canada is in a political muck with Russia it doesn't make sense to deny their help, Israel accepted Turkey's help to combat forest fire when their relationship was arguably at the lowest point shortly after the flotilla incident.
Governments should know when it's a good time to put a halt to their differences and emergencies are as good as an excuse as any.
P.S.
With forests being natural carbon traps climate change is now just as important factor as threat to communities and livelihood with the amount of CO2 were putting into the atmosphere we might not be able to longer afford for forest fires to take their natural course if we actually want to start reversing some of the changes we've made.
This isn't some clear cut case, and in this case it seems that Canada does take help from certain countries but for some reason refrains taking it from others and in such cases politics should really be the last thing on your mind.