I think grammatically the phrase "God bless America" is wrong; it should be "God blesses America", don't you think so?
God blesses America.
This is the Indicative Mood. It is just a statement that God (regularly) blesses America. The third person singular takes the inflection -(e)s.
God bless America!
This is the Subjunctive Mood. Here you are not stating anything. Instead, you're expressing a wish. The third person of the subjunctive coincides with the infinitive. Other examples of the subjunctive
God save the Queen!
Be that as it may, ...
It is important that he be here on time.