Farm Truck tags are a special break given to farmers. It allows them to save about 90% on the cost of yearly registration of a pickup or large truck.
They are given with the understanding that Farm Trucks generally are not used on highways as often as commercial trucks. In the case of grain trucks or other harvest equipment, they may only be on the road one month out of the year.
Because of that, Farm Truck tags cannot be used on pleasure or commercial vehicles. If you have a pickup with FT tags, you agree that it will only be used for personal farm business. The only allowed exceptions are school, church, doctor or grocery store.
I tag my pickup commercial. Thus I am legal when I pull a boat, take the kids to soccer practice, go out to eat, etc. etc. I'm legal if I use it custom farming or other business.
My grain trucks that I only use for my own grain, I tag FT.
I really despise jerks that buy $50,000 Lincoln Navigator pickups, Suburbans, etc. that never see a dirt road, and stick FT tags on them to save $50 a year.
Bottom line, it's a perk that farmers have gotten. Jerks that abuse the tag steal money from taxpayers, and may lead to the FT tag exemption being withdrawn, hurting me personally.