Q- the ham isn't allowed because its an agricultural/meats thing. Most countries forbid plant and animal products because of problems of disease. You may have studied the impact of the Dutch Elm disease in History class...if not, ask your History or Science teacher.
There are still countries that have been very sheltered and importing a plant that may have a disease can be potentially devistating... also importing an animal (live) like they did rabbits in Australia what disasterous because it disrupted the natural ecology. There rabbits didn't have any natural preditors and since rabbits reproduce VERY rapidly, it was havoic on the crops... believe it or not and caused a domino effect, leaving other animals and people hungry. As far as meat products, well, cured or not, remember reading or hearing about mad cow disease?! Different countries have different ways of treating and processing foods, some are cleaner and more sanitary than others... this can cause potential bacteria and other nasty critters to grow where you don't want them, and again, be detrimental to an ecology. Its also true that certain places simply don't have immunity to certain diseases, or natural preditors to certain insects or creatures- all of which could be very disruptive to the food chain- so better to keep these organisms out to begin with.
The cigars is a different story. That has to do with Americas embargo on Cuba. Just like legally we're not supposed to visit, we're not supposed to import anything from there. Until it changes, it will be illegal to have cuban cigars here :p
Or, if you like, its a conspiracy for the American government to:
A. Encourage americans to broaden their horizons and travel.
B. Deprive us of the better things in life.
You choose!