Being able to easily slide or slip the bag into the sizer seems totally normal and reasonable to me. What other way would you do it? I've watched people attempt to CRAM obviously overstuffed, EXPANDED bags into the sizer (failing, of course), looking like they are about ready to have a heart attack, out of breath but insisting they fly with it all the time.

This is why the problem will persist and it's not the GA's or even the security checkpoint vendor's fault. Many passengers refuse to take the blame or admit their bags are noncompliant. Some clerk in a department store or tag on the bag saying the measurements are okay does not make it compliant when you stuff all the sides to expand BEYOND the correct measurements. Keep it flush, keep the measurements clean, then you are compliant.