Because it is a large coal-fired power plant that emits large amounts of nitrogen dioxide gas that is partly responsible for visibility issues at the Grand Canyon and other parks in the West. The three agencies that issued the statement today are the EPA (Environmental Protection Agency), the Department of the Interior and the Department of Energy.
Gosar issued a statement explaining that the statement "seems to imply that the Obama Administration is abandoning a previous proposal that would have done little to improve haze while imposing catastrophic costs on the Arizona economy."
Here is the full Gosar statement, and the Administration's Joint Statement can be found at the above link.
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