I agree, but I would emphasize that they need to make the line CRYSTAL clear, and not just clear. When my bag is packed lightly, it fits in the tester-bin easily. When that same bag is packed full, it still fits in the bin, but it's a snug fit. I have to push down to make it fit.
A few times the agents wouldn't let me take it, with the same excuse in both cases: "If you have to force it in, then that means it doesn't fit."
So we hear the litany of "You bag must fit in the bin in order to board with it," but then even when it does fit, they start adding preferential exceptions that aren't spelled out anywhere in writing.