Actually it wasn't.
Originally Posted by Taylor
501(c)(4)'s could use general funds to engage in lobbying. They were subject to the Federal Lobbying Disclosure Act (LDA) and had to report how much they spent and how they spent it, but they did not (and still don't) have to disclose the source of funds used. So that was one way they could influence politics. They were also allowed to spend general funds to paying for partisan material, as long as it was only sent to their membership, and in some cases they were allowed to engage in ballot measure advocacy in certain states under certain situations.
But in order to engage in electioneering, 501(c)(4)'s had to set up a Federal Separate Segregated Fund (SSF) also known as a connected PAC. The 501(c)(4) could pay administrative costs of SSF through general funds, but the SSF was subject to the Federal Election Campaign Act. As such they had to report donors SSF donors to FEC using Form 3X.
However, Citizens United removed the restrictions on independent expenditures by corporations, associations, or labor unions. This included 501(c)(4)'s.
The Citizens United ruling did however remove the previous ban on corporations and organizations using their treasury funds for direct advocacy. These groups were freed to expressly endorse or call to vote for or against specific candidates, actions that were previously prohibited.