Said text was also translated into Aramaic and Greek so that all people would be able to read it. Remember, the language of the day for most Jews would have been Aramaic, and therefore many would have been incapable of reading the Greek, which would severely diminish the worth of the crucifixion from the Roman point of view, since those who passed by wouldn't know what Jesus had been percisely crucified over.

As for whether Pontius Pilate was being sarcastic, that matter is open to a great deal of debate. The Gospel of John itself says that in response to Jewish priest's suggestion that he write that Jesus claimed to be king of the Jews rather than INRI, Pontius Pilate replied, "What I have written I have written." (St. John 19:22, King James version)

Of course, that is fairly vague and by no means definitive. There are indeed many different views as to what sort of man Pontius Pilate was, and as to what he thought of Jesus.