This is not a duplicate of Why is FogBugz telling me “Cannot add case”? - even if it is quite similar.
Lets say I have set up a list filter which lists cases from Project A, ordered by milestones. Below each milestone line I can not add a case with the "Add Case" - so far everything is as expected.
Now lets assume that I want to exclude a milestone from my view. This is only possible with a "-FixFor:MilestoneName" search string, since the built-in Milestone filter only allows to choose a specific milestone to show, but not the other way around.
However, as soon as there is data in the search string, the "Add Case" is disabled. So (ignoring implementation details), it is obvious that the behavior should remain the same as before; there is not less information available to FogBugz in order to perform the quick add.
This is a general problem; as long as only axis are used, the information available to add a new case is quite often unambiguous. For instance, another use case is to add a "assignedto:me" (which works for all users if the filter is shared, in contrast to the built-in Assigned To filter criteria), where it is clear that the new case must be assigned to me.
Therefore it would feel "natural" that the search axis is handled just like a normal criteria where applicable, and that this doesn't block the addition of new cases.