Objectivism is a system of philosophy originated by Ayn Rand. When you add to it or change it, keeping the 'objectivism' label will make it tricky for people to know what exactly was part of the original system and what was added by future philosophers. Hence, Objectivism as formulated by Ayn Rand is a closed system.
When Kant's philosophy became popular, another famous philosopher named Fichte created a new system based on Kant's; however, while his approach was quite different (including dropping the 'thing-in-itself'), he claimed that his philosophy merely carries out the full implications of Kant's own ideas and that it maintains the spirit of the original. Kant rightfully repudiated these claims.
In the same vein, the brand of Objectivism proposed by the Atlas Society is not Objectivism, but an offshoot of it. Calling it Objectivism blurs the line between Objectivism, the philosophical system created by Ayn Rand (a historical artifact), and the modified versions. In this respect, the ARI institute is much more respectable because it always makes a point to mention when an idea is derived from Rand's system by another philosopher, but is not part of what Rand actually left in writing or publicly endorsed.