I'm guessing it's for aesthetic purposes, but here are two things I noticed:
Horizontal Line of Symmetry
1) All of the letters except 'A', 'T', and 'N' have a horizontal line of symmetry.
2) After rotating the 'A' and 'T's 90 degrees, they also have a horizontal line of symmetry.
3) 'N' doesn't get a line of symmetry, but the tilting brings it closer to having its center line be horizontally symmetric.
It doesn't explain why the first 'E' is flipped and 'N' remains an exception.
It's easy to see people or objects (i.e. weapons as mentioned by filistinist) in the letters after tilting them sideways.