That's why the MCAT exists--to provide some form of standard measure. One of the schools that you mention, Duke, actually posts higher average MCAT scores for their accepted applicants versus the national average for accepted applicants despite these Duke students achieving a lower GPA in a grade-inflated system. To be honest, Duke probably does practice some grade inflation but not enough to warrant the outrage that you're expressing.
The numbers for accepted applicants (2007):
Average Duke GPA: 3.55 Cumulative, 3.38 BCPM
Average National GPA: 3.73 Cumulative, 3.68 BCPM
Average Duke MCAT: 33.7 Q
Average National MCAT: 30.7 P
http://premed.duke.edu/wp-content/uploads/2007-annual-report.pdf
Thus, the fact that these students achieved a relatively lower GPA in a supposedly easier grade-inflated system must indicate that these students are huge slackers, and the system is incredibly unfair. Not only do they have a lower GPA, but they also do it at a place where grade inflation runs rampant. However, these same students tend to do better on the MCAT than the average accepted applicant does. So, it turns out that obtaining a higher GPA at Duke might actually be harder, and the argument that Duke students are on average smarter and/or more motivated might hold weight (if one assumes that the MCAT is a fair test of the intelligence and/or motivation of a student).