Dear Deubanks7525,
Thank you for your query.
In fact, in this specific case, an exception was made, since we really do want to enable customisation of this feature.
The code for the Revit MEP Calculation is provided as part of the product installation. In my Revit 2014 installation, it is located here:
C:\Program Files\Autodesk\Revit 2014\AddIns\MEPCalculation
I have not yet installed Revit MEP 2015 or Revit Onepbox 2015, so I cannot say whether it changed.
The source code for Revit 2014 has been cleaned up, refactored to provide a custom MEP calculation replacement add-in, and published on GitHub:
http://thebuildingcoder.typepad.com/blog/2013/07/user-mep-calculation-sample.html
http://thebuildingcoder.typepad.com/blog/2013/11/user-mep-calculation-sample-on-github.html
I hope this helps.
Best regards,
Jeremy