[quote=Jig]
That said, people are stupid and so should characters be. Characters that are perfectly accurate with grammar in speech and who never are somehow mistaken bug me.
[/quote]

Universal rule of fiction: Limited space to covey a hundred concepts. It's the same reason characters never use the bathroom, and any shower scenes or similar are there purely for character exposition or artistic liberalism, etc. The first thing to get summarily executed for the sake of space is the mundane, little things few would ever notice.

For instance: There are very few post apocalypse stories (though especially zombie ones) that openly ackowledge that after one year, all gasoline would be useless, and after five or less, so would most medicine. In fact, pretty much anything than good old alcohol and bandages would be done. Plastics would be fucked after twenty years or so. Kiss any non-brick or stone or steel building goodbye after 50. 

Suspension of disbelief is a potent tool for this reason. :hehe