I got a couple minutes so I am able to better explain....
Your assumption that the image is parallax free when the object is in focus makes multiple assumptions. The assumptions include, that the ocular focus was set correctly, that the optic system itself doesn't have major aberrations and lastly that the person looking through the optic has 20/20 vision.
For most hobby hunters/shooters, all three of those assumptions are most likely not valid.
1. Most people don't have their reticle focused 100% correctly, as evident by the repeated postings online asking how to set it, and the incorrect information that is passed on.
2. Most people do not have 20/20 vision
When someone with vision thats slightly worse than 20/20 goes to 'focus' the image using the parallax knob, he will focus it to the point that it corrects for his bad vision. This means that he will pass the 'parallax free' setting and his in focus image will NOT be parallax free. So now the user will have a clear image, but his point of aim will actually be different at every slight movement of his eye. Depending on the size of the target, the distance and the magnification used this could lead to bad accuracy and missing the target.
On the other hand, if the sight picture is set to be parallax free, even if it is out of focus, then the system will be dead on, no matter how you look through the scope.
This is the reason ALL reputable optic companies recommend you adjust the parallax out by moving your head side to side and seeing if the reticle moves. Notice that none of them say to just move the knob until the image is focused... You would think since that is the easiest solution, that they would say that if it was 100% correct.... This is probably the same reason that they moved away from calling it a focus knob and started calling it a parallax knob. That is its main function for marksmen. Not everything is a marketing gimmick. If you are looking through a telescope, the focus portion of it is more important than parallax and they call it a focus knob.
This phenomenon is experienced by many shooters all the time. You focus you scope to be crystal clear and then your friend sits behind your scope and starts playing with the parallax knob because the image is blurry to him. You sit back behind the scope and now its blurry to you. Its not magic. Focus is a based on the persons own visual acuity while there is only ONE correct setting on the knob for that particular distance that your target is at. So in this instance, one of you are wrong, or even both of you are wrong. This experience alone should show you that your argument that parallax and focus is the same thing is inherently wrong in REAL life scenarios.