tl;dr The IRS built an app for the iPhone. They built it using tools and libraries that hadn't internally been approved by the IRS. The team in charge of the app could have requested a waiver to use non-approved software, but decided not to because of time constraints. This was detected during a security audit. The audit was performed because this was the first time the IRS developed an iPhone app and they wanted to make sure it had been built securely.

The project wasn't criticized for using open source software or Objective C. They were criticized for using non-approved software and for not getting a waiver to use the non-approved software.