My surgeon wants me to take some antibiotics prior to my shoulder surgery next week. Why does he want me to take these when I do not have an infection? Is an infection expected to occur?  

You surgeon is taking extra precautions to ensure that you do not develop an infection.  The risk of infection is very low for an arthroscopic surgical technique and higher for an open surgical technique.  Depending on your history and the type of surgery you might have an increased risk for infection, however, rest assured that antibiotics prior to surgery is a fairly standard procedure.