Why can people not pick up after themselves?

It bothers me to no end when people do not pick up after themselves. I am not talking about my husband and his sloppy habits, after almost 9 years I have gotten used to that. I am talking about people out in public places leaving messes for others to clean up.
I have seen people leave their trays full of garbage sitting on the table at fast food restaurants. I have seen used paper towel on the floor in the women's bathroom, right next to the garbage can. At the Children's Above and Beyond Museum I have more than once seen the destruction a parent has let their child leave behind. These are usually things I see after the fact, after the person has left. However, this past Friday when I was at the museum I witnessed a father move his daughter from location to location through out the museum, and not once did he make her pick up before they moved on.
Now, I always give the benefit of the doubt. Maybe that was the way they found it so that was the way they left it or maybe he thought because other kids were around that they did not need to pick up. Since we were at the museum for a play date, I didn't let myself dwell on it. As we went around the museum the mother of my daughter's friend and myself made sure the two girls cleaned up each area as we went along. We are both members of the museum and we like to keep it looking nice for others to enjoy, and for the most part we made it a game so the girls were mostly cooperative.
When we reached the third and final floor at the museum, the father and his daughter were up there also. We spent a great deal of time up there and eventually found ourselves at the arts and crafts table. We ended up sitting by a nice old lady who is a paid volunteer through a program that helps find senior citizens jobs. As we were sitting there chatting we watched as the daughter took off the play clothes she was wearing and leave them in a pile on the floor. The father asked her if she wanted to go downstairs, she said yes and he walked over the pile of clothes and continued down the stairs with her. The old lady at our table told us that he is at the museum on a regular basis and he never has his child pick up nor does he pick up after her.
By this time I am highly irritated with the whole thing, not only because I find it rude to let your child make a mess and leave, but these are habits that my child is seeing. Children are little sponges and they notice everything and they mimic what they see, that is called modeling. As hard as I try, I can not seem to make cleaning up after ourselves a fun activity every time it needs to be done, but normally we get it done and my daughter know that is the way we treat the museum. This Friday when she said to me, "well that other little girl did not have to put away her toys" I was stunned. My friend Deb is the volunteer coordinator for the museum, and I told my daughter that the little girl who did not pick up her toys was being mean to Deb. Deb is an awesome woman, who is great with children and whom my daughter simply adores. This worked well for an explanation and I got no more grief from her. I was thinking to myself, if I ever seen that father again I was going to give him a piece of my mind.
Soon it was time to go, and as we made our way back down to the first floor to let the girls have a couple more turns down the slide, there was the father and daughter getting ready to leave. And guess what I did? Yup, that's right, nothing. I am not a confrontational person, and I know the museum has volunteers to help keep the place tidy in case they have customers like this father and daughter. But I vowed to myself if I ever seen them at the museum again I would find a way to be subtle and let them know they are expected to pick up after themselves.