Once upon a time, venue staff actually did work to promote their venues and events. They took care in cultivating an image for themselves in being a place to go to see new and up and coming acts. They did this through various means (some more than others) such as customer relations (e.g., being honest and friendly with customers, having decent selection of consumables and drink specials); physical presentation (keeping their place inviting visually [decor, etc.], sonically [i.e., decent sound system], comfortwise [furniture/fixtures, climate control, etc.], hygeine-wise [adequate rest rooms etc.]); regularly hosting well-thought-out events that didn't pander to the lowest common denominator (musical genre themed nights, DJs, bands, etc.); and adequate promotion (all of the above helps generate word-of-mouth promotion by customers, but supplemental promotion via posters, flyers, newspaper ads, working with promoters, etc. helps too). A good venue owner will make his or her venue a place people want to go in general; and not just rely on the efforts of specific bands to make crowds of people want to come out for one specific night.
Nowadays it's more likely that the people in charge of many venues do the bare minimum to stay open... They offer nothing exceptional to appeal to customers, don't put much effort into putting together decent/cohesive events or features, barely advertise (and when they do it seems most of the time they can't even be bothered to spell the bands' names right), etc. The bands are expected to do most of the promotion, advertising, bringing in paying customers, etc., and they rarely make any extra money for their extra efforts. Terms are often changed after the fact by venue owners, bar staff and bouncers are often cold or rude to patrons, and both performers and customers are left with a bad taste in their mouths, while venue owners try and maximize their profits by any means possible, both honest and dishonest, while putting as little as possible back into their venues and events.
Bands having to sell tickets to events held at venues they don't own and for events they didn't put together is such BS. Incompetant and lazy venue owners and promoters need to actually do their jobs instead of trying to run rackets. Performing artists should be considered investments, not desperate marks. Just as one might invest money in a good sound system, attractive bar fixtures, lighting, decor, etc., one invests money by organizing events and paying bands to play in order to help make the venue more attractive to paying customers.