It's true that sometimes I just don't want to be bothered while I'm running errands, but I wouldn't consider someone making small talk, cracking a joke in the elevator, etc. to be some sort of social transgression. As an introvert, chatting or small talk isn't really a chore, and I waste enough of my own time on my own time, so what's a few words with a stranger?
That said, it should be considered perfectly acceptable to signal that you don't want to stay engaged in a conversation if it's someone trying to explain their theory on building a perpetual motion machine. That actually happened. I did not want to stay in that conversation. But even extroverts don't like dealing with crazy.
It only becomes a chore when the talking itself is the work, such as dealing with Comcast charging you for a modem you totally did return, or dealing with the hospital when they incorrectly billed your health insurance and now they're trying to make you pay for the difference.