What types of work vehicles require commercial license plates in tennessee?

I was recently pulled over by a state trooper while I was out working. I haul a floor machine in my truck bed that may weigh a few hundred pounds. I would basically equate it to someone hauling a push mower as far as dimensions. He gave me a warning for my speed and told me I needed to get a commercial tag for my Toyota Tacoma since I was hauling the machine. I'm just curious if this is an actual law. I have been doing this type of work for probably 6 years and have been pulled over before and never heard any mention of my tags. Any advice would be great. thanks!