Is it illegal to have a dog riding in the back of a truck?

I was walking around an outdoor strip mall with my friend. As we were crossing through the parking lot, we saw a truck pass us by with a St. Bernard in the back, that almost flipped out the back of the truck because it was going insane. We went to go talk to the guy to inform him of our concern. We outright said, “we’re probably jerks for saying this, but your dog almost flipped out of the truck.” He went and tightened something in the back; (probably the thing holding his dog) and he rudely said, “he’s chained in, so mind your own business.” It was a single chain. The dog seemed aggressive, and wouldn’t stop barking, causing everyone to look and cause more commotion. This is Massachusetts, if that makes a difference with laws.