I spoke to consumer services with the company that makes my scissors. They do not magnetize the scissors at the factory, They become magnetic after they leave the factory, either in the shipping, in the stores, or most often in the home. After reading some of your posts, I am sure you already knew they can become magnetic after you get them in your home, but I actually remember buying one pair of my scissors, taking them home, and noticing them to be magnetic immediately. I can't explain how they got that way.
On the other hand, my other scissors that are magnetic probably got that way in my sewing room, because I didn't notice it for a while. It just seemed over a period of time, I started to notice they were magnetized as well. I don't use those as often, so it took a while to see.
Regardless of how my scissors became magnetized, it is really frustrating to use them around pins and needles, and I sometimes even find an attachment stuck to them! I am keeping my one pair of scissors that are not magnetic, away from those "contagious" ones!