A.G. Schneiderman apparently didn't read the US Constitution's fine print...
For purposes of clarity with regard to the article, one must keep in mind matters of scope and ultimate jurisdiction.
It doesn't matter what New York's A.G. office does, Attorney General Schneiderman can investigate until he's blue in the face. Even if his office comes to the conclusion that the AT&T/T-Mobile deal is a Bad Thing, there's nothing they can do about it except file complaints with the FCC (Federal Communications Commission), FTC (Federal Trade Commission), and/or DoJ (Department of Justice).
If they're really lucky, the state may be able to bring a lawsuit in the Federal Circuit seeking an injunction, presuming it doesn't get dismissed for lack of jurisdiction.
Why?
Simple: What we here West of The Big Pond call the "Commerce Clause" (Clause 3) in Article I, Section 8:
-- -- http://en.wikipedia.org/wiki/Article_One_of_the_United_States_Constitution#Section_8:_Powers_of_Congress
-- -- -- -- "The Congress shall have power... To regulate Commerce with foreign Nations, and among the several States, and with the Indian Tribes;"
and the "Necessary and Proper Clause" (Clause 18) in the same Article:
-- -- http://en.wikipedia.org/wiki/Article_One_of_the_United_States_Constitution#Section_8:_Powers_of_Congress
-- -- -- -- "The Congress shall have power... To make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers, and all other Powers vested by this Constitution in the Government of the United States, or in any Department or Officer thereof."
in combination with the "Reserved Powers Clause" of the Tenth Amendment:
-- -- http://en.wikipedia.org/wiki/Tenth_Amendment_to_the_United_States_Constitution#Text
-- -- -- -- "The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people."
Since a merger among large, nationwide-operating, publicly-traded corporations is arguably an Interstate Commerce issue
-- -- ("Congress shall have power to regulate Commerce among the several States")
any judgements blocking the proposed merger must originate at the Federal level. Furthermore, Congress has created Departments, Agencies, or Commissions to oversee Interstate and Multi-National corporations:
-- -- ("The Congress shall have power to make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers")
and has assigned the responsibility for regulating them to those Departments, Agencies, or Commissions:
-- -- (""The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.")
So, in short, the best that New York's A.G. office (or any state-level office) can do is offer an opinion, maybe bring a lawsuit.