They use English as window dressing because it’s cool. Most Japanese kids will take English in school, but few will ever attain any real fluency in the language. So if you sprinkle in some gratuitous English into your signage, clothing and television scripts or character designs, then you are cool, and you can still effectively communicate to some degree because while almost no one is fluent, they can still understand at least small amounts of English at a time. I’m not sure that still holds up with younger generations, but at least at one point it was the case.

If you visit Japan for any length of time, you will notice English phrases used all over the place, from restaurants, to street signs, to advertising and television, to the clothing people wear. I lived there for a few years right around the time Obama won the election in '08, and I encountered kids probably 6 years old who would shout "Yes we can!" when they saw me pass by. They tend to be more culturally aware and interested in Western culture (particularly American culture) than we do of them (which is saying something, given that Japanese culture is fairly popular in the West).