This question already has an answer here:
"We don't serve their kind here."
The bartender at the Mos Eisley Cantina makes it clear to Luke that Threepio and Artoo have to wait outside. With such upstanding citizens as the Jawas around, I know I wouldn't feel safe leaving my droids unattended.
Why then does the bartender (or possibly his employer) think it's okay to make potential customers leave their droids outside? What is the rationale?