Nevermind, I think I found the reason, you can still use:
if ($session->isActive) ...
and rely on the fact that failing to find such a method, class Component’s magical __get() method rewrites it to getIsActive and calls the proper method.
Still such design looks a bit weird. Why not simply name the method isActive in the first place?
It is a common practice in Yii to use properties instead of methods for getters and setters. getIsActive() creates a read only property when there is no setter.