For academia you have to go through a long chain of commands to get approval for new computer orders. Specifying that you intend for IT to upgrade your computer with ram that you buy from a third party would not pass along the lines of command. And if it would, you would have to justify it, formally with letters from consultants (A pain, and slim chance that it will work, due to dealing with multiple warranties and the like). In cases such as these it is simpler to pay the premium, as the processes is shorter and less hassle.
-I'm only referring to research labs at universities, the process might be different for other areas of academia.
-In your case, I don't think that I can find justification other than dealing with one warranty