To have respect for someone is fine. We tend to have respect for our sensei because we connect in some way along with acquiring a discipline like karate because the person worthy of respect provides us a service, the teachings of the discipline. There is absolutely nothing wrong with having respect for a person or a system such as Isshinryu.
What happens when a person or group transcends mere respect into "hero worship?" What happens when you put someone or some thing so far up they sit on a pedestal wearing a halo around their heads? What happens when the system becomes something that reaches a level of worship that it smacks of a religious type belief system?
One of the after effects of such things is stagnation, stagnation of the system as to its evolving with the people who practice and train it along with its growth with the times in which it is practiced and applied. It does not evolve as all things in nature must. It does not move forward but remains stationary with often antiquated ideas, beliefs and applications.
Modern beliefs based on antiquated systems tends to keep people under the influences that come from power over others. It manifests as a closed tribal system where they other those who don't belong and keep their doctrine close to the social standing within the tribe. When people gather around such a person or system looking up with wonder and awe in their eyes and on their faces we tend to think, "cult."