Mad as a brush

So, aparently if you’re American, and you see this sign, its a very funny thing.
And the whole ‘mad as a brush’ saying doesn’t translate too well either. Mad meaning angry rather than crazy and all.
We really should have taught those Americans English properly before the Boston Tea party had chance to happen. But maybe there was no hope in the first place. After all, who in their right mind would dump all that lovely tea into a harbour. They could have knicked it and drunk it at least…