My guess is that the folks bitching about you guys are extrapolating from the "standards" that 18f produces, and spinning that into the RFP thing.
Contractors hate standards, because they scope what they can sell. If you make a simple statement like "use Git for source control", the vendors want to be able to contest that decision, or have the decision made in some sort of "open" form where they get to lobby for some other standard.
*I work for 18F.