The necessarily incidental concept applies to legislative powers. All that means to say is that the federal government can exercise legislative powers in a provincial area of jurisdiction where it is on a matter that is necessarily incidental to some area of jurisdiction they already have, like airlines or banks or whatever. Normally they can't legislate labour, but they can legislate labour when it relates to a federally regulated industry or business. That's all “necessarily incidental” means. It's relating to legislative powers. It's got nothing to do with the spending powers of the federal government.
On October 18th, 2011. See this statement in context.