Also, being polite shouldn't be a conscious decision. You should naturally say "please" and "thank you" and you should instinctively hold doors open for people. These things should be second nature and you should do them without thinking.
You should say "please" and "thank you" to AIs for the same reason you should use your turn signal when no one is around: so that you'll also do it when you're stressed and in a hurry and forgetful and won't accidentally be an asshole.