If I look in Preferences>Audio>Driver Settings, under "Recording Timer Master" I have "UA-25EX IN", which is correct for my little two-input Cakewalk UA-25EX interface.
When I hit the Echo button, my [ Input] dropdown automatically changes from "None" to "SterU25XIN". If I hover my mouse over the relevant dropdown, the yellow tooltip reads "Input = Stereo UA25EX IN".
If I set the Echo to ON, Sonar needs to get the signal from somewhere. If it is set to None, I guess the design committee decided the first available input pair is a reasonable default source.