Here's the analogy I tweeted a couple weeks ago: "SOPA is the equivalent of allowing Disney to shut down a restaurant because a customer drew Mickey Mouse on a napkin."
I think anyone can recognize the injustice in that.
The shoplifting analogy seems more immediate and straight-forward to me. I understand that the shoplifting analogy equates piracy with theft but the distinction isn't really all that relevant to SOPA. Uncle Art probably equates piracy with theft anyway, so why fight the battle on two fronts?
One final point on this, by indirectly conceding the piracy/theft point you're less likely to step out of alignment with Uncle Art thus making it easier for him to agree with everything else you say. :)
P.S. I know we're all probably focusing too much on that killer soundbyte but I do think it was the missing ingredient. :)
If we abstract analogies too far, I worry that people will ignore them. I tried to leave Uncle Art with very few dots to connect by only changing the "host" business in my analogy.