Perspectives sometimes allow quite complex views of tasks that can be contained in different lists (projects) and folders, that must be assigned to certain tags and/or that must have a current due date. Projects can also have a non-simple structure (tasks contain sub-tasks, sub-sub-tasks, order of tasks is extremely important).
Therefore, assigning tasks to projects in the perspective view does not make sense if complex perspective rules do not display a structure of projects.
Ok, that agrees with my assessment of the current situation. Of course it’s not an unsolvable problem for Omni. Based on the rules of the perspective, the new task’s properties could be automatically set before/when the new task inspector is shown to the user. (The user could still change anything, which might result in the task disappearing.)