Simon, I understand what you are saying completely, including the difficulties presented on the island. I just don't agree that it justifies stealing.
What other product would you apply these arguments to?
Sounds like good tuna is difficult to find and expensive when it is available. Does that then justify pocketing a can or two next time you're in the mood for it?
Your argument almost makes it seem there simply are no other options. There are; they may be more difficult and the results not quite as desireable, but it beats stealing.