"Act of God" in this context is a legal term. It is not to be taken literally. My
Oxford Dictionary of Law defines it as "An event due to natural causes (storms, earthquakes, floods, etc.) so exceptionally severe that no one could reasonably be expected to anticipate or guard against it." It is also defined as in Tennant v. Earl of Glasgow (House of Lords, 1864):
Circumstances which no human foresight can provide against, and of which human prudence is not bound to recognize the possibility, and which when they do occur, therefore, are calamities that do not involve the obligation of paying for the consequences that may result from them.
Now, I confess I rather like the term, because it confines "God" to a purely metaphorical usage, which is all the word is really fit for. But I think the use of the phrase as a legal term might be on the way out. Anthropogenic climate change has already blurred the distinction between "natural" and "human-made" events, and that's only going to get worse. Insurance companies and lawyers are going to have to deal with the outcomes of apparently "natural" calamities that human foresight could and should have provided against, and of which human prudence could and should have recognised the possibility, and that should involve the obligation of paying for the consequences. If it wasn't so depressing it would actually be quite fascinating.
Emma