Pulling a Punch Line

A store near the baggage claim are of the Rome airport has a handful of signs, about hip high, in the entrance, each with the image of some famous person and a quotation praising books. Some are to be expected, for example Thomas Jefferson's "I Cannot Live Without Books". One did surprise me:

I considered the reasons one might provide only half of the quip:

1. Not enough space. But Oscar Wilde gets more words.
2. "Inside of a dog, it's too dark to read" just doesn't work in Italian humor.
3. The signs are there to encourage one to buy books, not to make one laugh
4. "All'infuori di" has a logical meaning but not a spatial one, and so cannot be opposed by "dentro" as "outside of" is by "inside of".