|A minor worker rides on a cut leaf|
In many habitats, these ants are attacked by flies in the family Phoridae. Female flies alight on leaves in transport, crawl towards the carrier, and lay eggs on the unwitting ant. When the eggs hatch, they enter the ants body and consume it from the inside out, eventually killing it. Hence the reason for the seemingly lackadaisical passengers. A fly is much less likely to land on a leaf with a rider, and if it does it will quickly be chased off. The minor workers are defending their sisters from aerial parasitoid attacks.
If you are worried about flying parasitoids, for a small fee I will ride around on your shoulders for a day, defending you from various forms of death from above.