It could also allude to the subjunctive mood if you think of it as being implicitly prefaced with "May God help us", since the present tense subjunctive is always just the bare root of the verb for every perspective.
'God helps us', singular 3rd person simple present verb form, would be a statement of fact and belief, a truth, according to the speaker's personal convictions.
'God/heaven help us/me/you/them!' is an intense imprecation which might be used by people who do not believe in any 'god' or 'heaven'. It's a common figure of speech.