If you see a fish in a fish store that has what appears to be ICK or any other thing like cloudy eyes etc and the tanks are all set up side by side like most stores do. Wouldn't the way the tanks are filtered and the water returned just spread to the other tanks. I.E. if they all shared a big common sump?
Is this why people do not QT or observe a new fish in the sump instead of using a QT tank?
That is exactly why. If the infected fish is in your sump, the parasite will make its way to the display in short order.
Quarantine is just that, quarantine, you keep it away from everything else, in a controlled environment, so that you don't infect a healthy population.
Some fish stores have isolated systems to allow for treatment, but space constraints or associated costs make it hard for others. If you happen to find a LFS that does QT, stick with them, it's a good store that cares about what they are doing!
QT / Hospital tanks are a great investment in the future of the hobby. Everything (chemistry, filtration, supplementation, etc.) is already hard enough, why throw ever dying fish into the equation when you can fix it with a $40 setup from craigslist....
I'm an avid QT supporter for the simple reason that I hate fish dying, after I adopted a good regimen my fish losses all about stopped.