Um. Maybe I’m not fully understanding the author’s intention, but the sign (er, piece of paper) they put on top of the ad really isn’t at odds with the ad. It’s a bit ambiguous, scratching out “prob” and covering “bly no go” with the sign (“there’s a D?”) But the message that love is the most important thing is something most atheists would agree wholeheartedly with. With no supernatural being demanding faith, of course love for others is the most important thing there is. It’s all there is.
Some commenters at BlogTO suggest it’s a hate crime. But it’s more of a love crime. It’s like smashing a church sign then spray painting a cross on it.
Maybe the vandal is just confused. Or maybe it’s deeper; maybe they are pointing out the common belief that atheists, religious folks and The Beatles agree on: all you need is love.