ABET may have something to do with that: There was something big that happened in 1980.
http://www.abet.org/history.shtml
Our 3.0 policy is because part of the new hire process requires that you et your MS at night and most schools require a 3.0 to get your MS.
I can speak for earlier times, but I grauated from Purdue in 2001. By that time about 50% of employers I looked into wanted a 3.0 or higher explicitly. A couple even wanted a 3.5 or higher (but those were rare and mostly for research firms).
As far as the 3.0 thing goes, it just keeps a lot more doors open. I had friends with 2.5 GPA that still got nice jobs, but I also know that they got turned down on the spot for many jobs they wanted.
Purdue is a really good engineering school. Often top 10 depending on the degree. I have heard of that grades don't matter if you went to a certain school, but I have only heard this referenced to Ivy League and MIT.
P.S. At Purdue there were several courses that believe in extremely tough exams i.e. median scores of 30%. Typically though these scores were normalized so that the median grade was around a 3.0.