This issue, which has played out countless of times on this forum (and will continue to do so in the future), almost always ends up at the final common denominator of: "Where do you draw the line?" (as Quentin has adroitly pointed out above).
It's easy to create hypotheticals that will twist and confound almost any ethical stance as black/white moralistic concerns confront reality.
Is it lost income that's important? How much we like/hate the alleged perpetrator (or convention sponsor)? The legality? The principle itself? Such arguments typically involve all manner of biases, belief systems, ethical constructs, and even nationalism. Heck, even Dr. Suess gets some play ("Stealing is stealing no matter how...")
I rather like Helder Guimaraes' stance referenced earlier as I adapt it in my mind to pertain to this issue. Namely: sure you can get away with it, but why would you want to? What does that say about you as a performer? As a person? Can't we, I, do better?
That said, IMHO there needs to be room to construct such routines as the brilliant one published by Max Maven in Genii a couple of years ago involving movie posters. In it, he recommends finding images of movie posters in a book or on the web to print in the required size if one wishes to perform the trick.
Is Mr. Maven recommending thievery? May it never be! However, what some might term "fair use" causes others to scream "Stealing!"
All of the above, doesn't mean such discussions aren't worth having. For me, though, it does mean I've come to develop a bit of charity towards thoughtful people who disagree with my particular stance on this topic as I consider their own unique points of view.
We now return you to the regularly scheduled bromance still in progress...