After hosting events, and once everyone's been paid, it's easy to forget (or rather, attach little importance to) removing advertising posters. While [I find] they often have similar aesthetic appeal to graffiti, for the sake of keeping the city tidy it's important to remove old advertising posters once events have finished.
By-laws governing the city allow the municipality to recover costs for the removal of posters from the individuals or organisations hosting the advertised event - but even though this is the case, I have my doubts that this is often enforced (probably because there's so many other things to take care of). Regulations regarding posters can be found in Section 11 of the December 2001 Western Cape government gazette. It's probably a good idea to review the document if you're in the event industry. ;)