This isn't a browser feature, it's an AdBlock Plus feature. You're just switching off the blocking for certain sites, not turning off the extension itself in any meaningful way.
Is this a new thing? Most of the time when I've installed extensions I've just seen an initial prompt that says the extension can view and modify your data on any page, so either agree & add, or get out.
But you can select on which sites the extension is active. You can even make it only active when you click on it.