As more revelations surface about how the Internal Revenue Service singled out conservative political groups for special scrutiny when they petitioned for tax-exempt status, it's worth asking why the IRS chose to lump all of these organizations in the same category.

1040 tax forms. Bloomberg photo.

That move, it turns out, is part of a longstanding IRS practice aimed at ensuring similar organizations receive identical tax treatment.

Marcus Owens, who oversaw tax-exempt groups at the IRS between 1990 and 1999, said the agency has traditionally categorized groups with common characteristics seeking the status of a "social welfare" group together -- whether it's hospitals or churches or political actors.

For example, the agency has been under a permanent injunction since 1970 to give special scrutiny to any primary or secondary school in Mississippi seeking tax-exempt status in order to ensure the institution is not engaging in discriminatory behavior. The IRS applies that same principle to primary and secondary schools in other states with a history of segregation, Owens said, such as Alabama and Louisiana.

"You group them together by the same criteria so there's consistency and employees have consistency in the outcome," Owens said.

The IRS is using the same sort of approach when it comes to scrutinizing how hospitals are complying with the new requirements of the Affordable Care Act. Of course, the ongoing controversy has deepened conservatives' suspicion on that front as well. "It adds to the animosity to Obamacare," said Sal Russo, chief strategist and co-founder of the Tea Party Express, a political action committee, "because the IRS plays a big role in implementing Obamacare."

The thought by the IRS is the right one: Group like things so they all get judged by the same standard.  Of course, when you start singling out individual groups based on their names or their stated missions, you veer from the right idea into the wrong one.