I completely disagree. Griefing someone is harrasing a player when you have a strong advantage, be it in level, gear, or number of players. Whether intentional or not by the devs doesn't matter.
The amount of time it takes to fill a whole cart with goods is quite a bit and whether you succeed or not shouldn't be decided on whether or not someone decides to rob you. There should be mechanics involved to negate that risk, like hiring bounty hunters or a delay in enemy respawn. Anything to make it a more fair playing field.
You are right about a posse. But I've found no one wants to take the time to help a trader out as the rewards for them are not worth the time. Unfortunately.
The mechanics are broken because one side takes all the risks while the other reaps all the profits causing gamers to only do local deliveries. If the devs intention was to create a scenario where players completely avoid doing a part of their game then they succeded.