This is a common misunderstanding of Citizens United. Businesses and corporations have always been allowed to engage in politics. They just weren't able to do it with general funds, instead they had to form PACs.
As to the IRS approving organizations that should not have been approved, this has only become a problem since Citizens United. If you wanted to form a 501(c)(4) for issue advocacy, so what? There was no benefit over a PAC so there was no real reason to apply too much scrutiny.
After Citizens United these 501(c)(4)s emerged for the sole purpose of laundering dark money into politics. For example, CrossRoads GPS spends more than 80% of their budget on adds, but only declares a small portion of them to be independent expenditures.
Meanwhile, an apology for these IRS transgressions isn't enough.
"He who does not think himself worth saving from poverty and ignorance by his own efforts, will hardly be thought worth the efforts of anybody else." -- Frederick Douglass, Self-Made Men (1872)
Citizens United basically said that you can't prohibit or restrict corporations/unions from engaging in "independent communications" i.e. independently advocating for candidates in an election. It is because such expenditures were legal under a 501(c)(4) prior to C.U. that they couldn't take corporate donations. After Citizens United, the restriction was lifted. Thus, Citizens United didn't change how the 501(c)(4) worked, or who was shielded and who wasn't, it only allowed corporations and unions to make use of a 501(c)(4).