The reason is simple. Even though they have native teachers there, they don't use them. They put their info in Google Translate and use it. On websites, billboards, posters, in books, and on paper, they have tons of mistakes.
It wouldn't be too hard to remedy this. All they'd have to do is ask the native speakers to help edit any and everything in English. Better yet, hire someone specifically to do this. People need to swallow their pride and ask for help. If native speakers have people proof-read their work, then non-native speakers should as well. It'll make your school look better and people will stop laughing at mistakes and putting them up on Engrish.com
TEFL Tips recommends: