Well it's kind of open to interpretation, which may be why they didn't want to directly say that, just imply it.

Article 19 of the Geneva convention:

The protection to which civilian hospitals are entitled shall not cease unless they are used to commit, outside their humanitarian duties, acts harmful to the enemy. Protection may, however, cease only after due warning has been given, naming, in all appropriate cases, a reasonable time limit, and after such warning has remained unheeded.

Now are firing qassam rockets "harmful to the enemy"? Probably.

Has due warning been given? Maybe? It's not well defined what that means. Does roof knocking count? Do you need to submit a form to their embassy?

I think the big problem is that the kind of warfare we are seeing here is unlike what they saw when they wrote those laws.

It is written in the passive. That's intentional; it's a classic approach that writers use to dodge the issue. It's not OK and we shouldn't excuse it.