Like the PP, I had the easiest baby ever, but no way would I have every taken her for such a long appointment.
What would you do if baby starts crying while you are getting your hair washed out, for example? Of if you are mid coloring and baby has a meltdown? You can't just run out the door.
I also think that many people go to the salon to get their hair done and enjoy the break, and they don't want to deal with someone else's baby. Some places just aren't baby places in my mind, like fancy restaurants, salons, spas, etc.
Time is money, and if you have to take a break to deal with a bottle, it can delay your stylist, and future clients.