As someone who has directed many races before, here is my perspective.
Races must fairly provide the same advertised amenities for back of the packers as for the race leaders. Same aid, same course markings, same finishing medals, etc.
That said, it takes extra preparation and resources to properly accommodate back of the packers. Often times these people are more likely to have marginal fitness and other health issues than those nearer the front of the pack. This can tax a race's emergency response network. Back of the packers often require more aid (water/ gels) because they are out there longer. There must be adequate sag wagons to evacuate DNFs from the course.
Prudent race directors establish a time limit under which all official finishers must finish, and intermediate time cut offs to ensure participants are making adequate progress towards finishing under the time limit. These need to be clearly stated from the beginning as part of the initial race information.
Clear directions and information must be provided to runners regarding what happens if they do not make the time cut offs.
No matter where a race director sets these time limits, there will be a group of runners that will be unhappy because they can not finish under the set time limits.
Also, anyone who is not satisfied with the way current races are conducted is welcome to become a race director.