Around here it's the DOT and what kind of mood they are in. Our ratio of coal trucks and timber trucks are probably much higher than normal and we've got an abundance of DOT. I've had trucks pulled over 3 times this year, just to be nit picky.
I honestly don't think that having push mowers in the back of a pickup is a big deal, as long as the gate is up. Around here what gets most guys is that the law is anything over 8K has to have special tags. I can't count the number of guys I've heard with 1 tons pulling small tractors, skid steers, etc.... who have gotten $400+ tickets for being over their tag weight.
Whether or not it's a law, I think you should always be on the side of caution, especially if you think there is a chance that something may get loose and get out. Lawyers always find the loopholes. That's what your liability insurance and such is for. Be safe, and practice CYA.