Different universities have granted approval under different reasons. At this writing, eight researchers have gained the approval from their home institution's IRB and they fall into four categories.
Non-Human Research
This has been the most common exemption used by
Vanderbilt University, Nashville, TN
Harvard University, Boston, MA
University of Maine, Orono, ME
Univ. of Colorado, Colorado Springs, CO
Survey and Testing
Carnegie Mellon University, Pittsburgh, PA and Southern Methodist University, Dallas, TX classified their studies as exempt under number 2 (§46.101b4) (the "survey and testing" exemption that also claims to don't know personal identifiable information).
Normal Instructional Practice
The University of North Dakota, Grand Forks, ND classified the study as exempt with #1 (§46.101b4) (normal instructional practice).
Anonymous Data
Teacher College at Columbia University, New York, NY used exemption #4 ( §46.101b4 which states "(4) ... if the information is recorded by the investigator in such a manner that subjects cannot be identified, directly or through identifiers linked to the subjects." ).
Finally, if you need an example, email nth@wpi.edu and I can share an example.