"It would be wiser" and "it's not that hard" suggests the fans knew what the flag means to some and thought "sod it - we'll do it anyway".
It could be the case that they hopped onto Amazon typed Japan flag and happened to choose that rising sun type thing as it's a bit more eye-catching -...
I had no idea what the Japanese Imperial Flag was before reading this thread and opening another browser tab and searching.
I'd probably have described it as "that Japanese rising sun thing".
I would hazard a guess that the Albion fans who had this flag may have thought of it in a similar way...