I think the assumption is that MBA students are more ambitious than normal. Students often choose the MBA degree because they are unsure what they really want to do. An MBA is seen as a generally useful degree which could lead to a high paying job in many different fields. Limiting the study to students who had made that choice, for whatever reason, could very well have an effect on the study's outcome.
I fear instead the MBA and post-undergrad route has become the new "safe way to go". Extend your student loans, stay with your friends from college, keep going out on the weekends, keep a relative life of leisure (subjective, the course loads of some people are egregiously harder then others but from what I've seen it tends to be similar to undergrad for most).
