From my understanding it isn't the fact that it is a super-PAC per se. Rather it is the involvement of the RNC and Republican candidates with the super-PAC.  From your link (emphasis mine)

Four groups not connected to the candidates or the Democratic Party, one group each for the House, Senate and presidential races, and group number four, called American Bridge 21st Century, founded by progressive media watchdog David Brock. Its mission is to set up a war room and, says Harris, do opposition research for the other groups.

And from TPM article:

First, this constitutes an illegal scheme to violate the ban on the raising or spending of soft money by national party committees. Second, the proposed activities would violate the ban on federal officeholders soliciting unlimited soft money donations in connection with a federal election," they said. "Each of these bans has been upheld by the Supreme Court, and neither of them was affected by the Court's decision in Citizens United.

Here's some more background on where this is leading from the Columbia Journalism Review.  From Rollcall quoted in the article:

Democrats have railed for more than a year against the landmark Supreme Court decision that unleashed unlimited and undisclosed corporate and union dollars in political campaigns, but now they’re preparing to collect and spend this same money.

Democratic operatives are racing to organize new groups to solicit and spend millions of dollars that the Citizens United ruling allowed, gearing up to play by the same rules as Republicans regardless of whether they like those rules.

They all insist that they don’t. But after watching Republicans take advantage of the new rules to spend unprecedented volumes of cash and win House and Senate seats across the map in the 2010 midterm elections, they say they can no longer stand back on moral grounds.