Matt's answer is right enough, and Axioplases's description does have historical accuracy, but I felt differently enough to propose another answer.
First, here is the truck in question, with the words
カンガルー便 written on the side, "backwards".
Note, though, that the text for the parent company, Seino, is the "right" way round, presumeably because it's in romaji, which is not as flexible as Japanese kanji and kana in terms of direction. If this were simply a matter of viewing the car in a "front is top" concept, then why not also include the romaji? It's only the Japanese text that is flexible enough about direction.
How flexible is Japanese in terms of direction? A little. The root of the issue lies in the fact that Japanese text is traditionally written vertically. The choice to go left or right when writing horizontally is, or was, therefor somewhat arbitrary. Back in the day, one would have come across writing right to left more than today, as in this old train sign:
Or how the car brand "Ford" is written
ドーオフ) on this Taishō era building:
Again, note in both pictures that the romaji offered goes left to right, but both kanji and kana are right to left.
Why do these historical examples matter? Because it's evidence that there is a cultural basis for accepting text right to left that makes the truck sign possible in Japanese culture. Contrast that notion with a hypothetical delivery company in an English country that wrote "Yreviled Ooragnak" on the side of their trucks, because they wanted to follow a "front is top" logic. It just wouldn't fly, because the horizontal direction of text is locked in for our language.
Of course, the massive exposure to English text has influenced textual presentation in Japan, to the point where you don't see this much anymore. But it's not a law that writing must go left to right, and some people will still go right to left, like this racist jerk here, who probably did it precisely because he doesn't want to play by the west's rules (I feel a little bad using this as an example - it makes my point, but the content of this guy's sign is unredeemable):
So the "backward" phenomena is not just about vehicles or flags, or a front-as-top logic. Japanese text, insofar as it has escaped English cultural hegemony, can be flexible about which horizontal direction it goes in, though admitedly it's rare, and probably dying out. (Except with right wing jerks).
What I'm pretty sure you won't see, though, is bottom to top. The text direction is not that flexible.