Well of course a customer can ask for whatever they want and demand whatever they want and throw out whatever proposal they want. Fine, but it’s “their loss.” I’m just mystified why some organizations operate that way, but they do.
When dealing with monolithic organisations you have the choice to do it “their way or the highway” so the post linked above is typical and, frankly, a good pre-amble to going into any public procurement process.
It might even make you laugh.