Well. You think what you like. It's good to replace all kinds of things. its good to have new stuff. But hes replacing a damaged caliper. Not a frozen one. And rears are much less prone to freezing that the fronts. And just because you think one will go bad down the road is no reason to replace it prematurely. Is it BEST practice to rpeace both calipers? Sure. Is it necessary. Not at all.
By your logic (they are part of the same hydraulic system) he should replace all four calipers. And what sense does that make? We don't drive race cars for a living. We drive 4x4 grocery getters to and front work and then on the weekends we turn wrenches to get our jollies. Don't over think it.
Think he means that it takes two to tango. just like changing one damaged tire, you should change the one on the opposite side as well to balance it out. for safety, security, and peace of mind.
Your logic is trying to save every dollar down to the penny unless the issue comes up when it should be done as preventive maintenance. Just like you change your oils in a timely manner before your engine decides to crap out, etc. You don't wait till there is a problem, especially your brakes. Common sense.