Yeah, it seems like a bit of a loophole in the system. In theory setting one or two defenses should increase the likelihood your opponent will defeat you, and you've sold yourself short on getting the defensive banners.
However, if there's one squad your opponent can't beat, well, that's a way to take advantage of it. But that's also open for debate, as if you don't have that squad or the counters to that squad, what have you been doing with your roster and resources. If you have a counter but set it on defense despite being able to preview their roster beforehand, then there's the issue of defensive/offense balance strategy. So someone will likely come out and tell you it's totally fair.
Mind you, there aren't many people on here saying that the other person robbing you of the chance to gain credits is an ok design. That is definitely the main problem with the set up and the way the auto defense currently works.