My view is, there were no ties on them, they were placed in a bed of wheat (which was also shipped around the Med, but not economical in and of itself) -- hence the pointy end, which made it practical to extract the wine shipment piecemeal. After set in place, the jars were 2/3 buried in wheat. AFAIK, there's no evidence for tying them, and the wheat would have kept them stable, kept them from cracking in rough seas, and would have left them available. And the wheat would be also a saleable product.
There’s the additional aspect to their design for loading — similar to barrels, a great solution for transport.