I stand out of respect for the people that do stand and say the pledge. To not stand is to say "what are you stupid people doing". It is the same when I am at a family function where a prayer is said before a meal. I am the only atheist in my family but I bow my head and remain quiet while the prayer is said, out of respect for the people who are praying.
i do not have respect for sheepeople that stand and recite it - it is a dangerous thing that leads to demagoguery