Complex sentence: At that time in San Francisco, the ticket-a-dance system was used in what were called closed dance halls, because female customers were not allowed: the only women permitted in these halls were the dancing female employees.
Split into simpler sentences: At that time in San Francisco, the ticket-a-dance system was used in what were called "closed dance halls". They were called closed dance halls because female patrons were not allowed--the only women permitted in these halls were the dancing female employees.