I've seen confidentiality the other way (where a vendor can't say who their customers are) but why would someone not want a customer to tell the world how great their software is?
Only reason I can think is that if the customer messes up the implementation and creates a subpar service with it then it reflects poorly on the source provider.
Actually another reason might be that the source provider doesn't want other users to know about each other to compare licensing agreements and such.
I'd guess it was written by a lawyer who was more concerned about a customer telling the world how great their software isn't. And a blanket "do not mention" statement is probably safer than a "do not mention negatively".
