What is "impure" about beet and cane sugar? They are all plant products with similar concentration processes. What a specious argument.
I think he means pure as in no additives, sugar in this case would be an additive. Sugar by itself is pure.
Adding cane sugar to maple syrup would make the the maple syrup not pure. And adding maple syrup to cane sugar would make it not pure cane sugar.
Again, nothing was said about cane sugar being impure. And the FDA doesn't appear to be arguing that evaporated cane sugar needs to note 'added sugar', only that evaporated maple syrup needs to do so.
Probably need to look at folks making things like Mrs Butterworths (slightly) maple flavored syrups. Some such lobbyist wants better comparison to the real stuff.