Niagara Falls has a hugh hydroelectric power plant. Don't it?
So do many large dams, which if taken out would not only knock the power off that they provide but also cause catastrophic flooding downstream. And people who relied on the water supply of the lake above might get thirsty.
As someone above suggested, maybe its symbolic for water supply? I dont know Niagra well enough to have a valuable opinion really. But it seems like a less desirable target to me.