This is because && and || are often used as conditionals:

    [ -e README ] && cat README

avoids an error if the file README doesn't exist, and

    [ -e README ] || echo "You should write a README!"

works the opposite way.

What's more pernicious is that pipelines don't cause the shell to exit (assuming set -e) unless the last command fails:

    grep foo README | sort

does not fail if README doesn't exist, unless you've also used `set -o pipefail`.