Ehhhh... a lot of Japanese people know a functional amount of English. Like, not enough for fully, grammatical questions but enough that you could ask "Where bathroom" and get a proper answer. With that in mind, the English language is honestly the language of the world and, when you want your protests to be seen by the world, it would make sense to include English words. Not to mention that kanji takes up less space than English words to mean the same thing, so you really couldn't have a large banner.