Or they are just repeating crap they have also learnt from other varying sources and not found out for themselves....
Serving people would obviously get bad knees from the weight, were talking airsoft here where people are hard pushed to wear the gear they own for any longer than 6 or so hours (even milsims I would say the same as people slope off for food, drink etc). Me personally, I have a set and I also have quite bad sciatic pain (again this is me personally) since getting a set and running them in my plate carrier its helped with the issues as my PC actually sits where its supposed to and is more evenly balanced after having a long hard look at where and how I had set up the externals, pouches etc. Yes, training plates aren't for everyone but as @BibbsOnTour mentioned they come in varying weights from 1.6kg upwards (and judging by majority of airsofters who carry way to much unnecessary shit anyway people can hardly talk about carrying to much weight). People actually thinking about their kit and setting things up right would barley notice any difference. hell I had a guy this weekend adamant he was taking three guns out to each game, two of which... guess what... stayed by the spawn the whole time.
This ^
Instead of bashing the people that do have them or want to run them in whatever weight they choose why not be productive and ask questions about them and what benefits they have found from using them, again yes armed forces wear plates. They also do a lot of other things that airsofters don't do, like tab for insane amounts of hours in blistering heat not only carrying plates but ammo, weapons systems (that are a lot heavier than airsoft guns in most instances), water, radios, food, med kits.... the list is pretty long, so while I appreciate the weight of plates is adding weight to you i would hazard a pretty fucking big guess its not just the plates that would account for knee issues within the military... happy to be proven wrong if plates are the sole reason for bad knees.