Here is my take. I tip because it is the social norm to tip if they handle my bags because yes, it isn't their job. What I don't get though is if it's not their job, why do they do it every single time without asking me if I want them to? If I go get my own bag and I don't tip, im the jerk for not letting them do it so they can get a tip. I'm not a flight attendant, I don't have 4 bags on a 2 day. I pack light, I don't need you to handle my 10 lb bag, it's not worth the dollar to have them do that every time.