Like Mule Drug Does What
There have been cases where grandparents were caught being mules. It's a matter of risk versus reward - if people feel that the money or the threat justifies the risk, they will be a mule. Jail time for first offenders is usually small, so those are the kinds of people that most drug runners want to use. Consider the OP - they were on a cruise ship, delivering drugs into Australia. They owed the dealer over $20k, and the dealer probably paid for the cruise, so they saw this as an easy way to get out of the debt and have fun. I wish my mortgage bank would work that way!