And if you openly pirate, say, Steamboat Willie, on the grounds that it should have passed into the public domain, I think that's a reasonable and arguably justifiable protest. But if you openly pirate, say, Wreck-it Ralph, or Brave, or (shortly) Monsters University - movies that have been out for less than a year - then your alleged argument that copyright should have a shorter, 20-30 year term is unsupported by your actions.
And therein lies the problem... How many people are pirating Steamboat Willie, the Seven Year Itch, To Catch a Thief, etc., as opposed to recent movies, music, games, and software? The Top 10 most pirated movies chart for this week lists Life of Pi, Silver Linings Playbook, Skyfall, Django Unchained, Argo, The Hobbit, Here Comes the Boom, Flight, Zero Dark Thirty, and Robot and Frank, every one of which is less than a year old. Similarly, the top pirated games (for 2011, I can't find 2012 numbers) were all released that year.
Your suggestion isn't bad, except that people shouldn't be pirating just anything, but specifically things that would have fallen into the public domain but-for the last extension. Things that are currently 60 years past the life of the author, say. That would be a meaningful, and credible, protest.
I agree with you that there's a difference, but...
I'm reminded of a story I heard somewhere (book, movie, sorry can't remember) were some people were on their way to work and they where discussing the fact that if they were late the punishment was death. Somewhere along the way they were delayed and were going to be late for their work and one guy asks another one what is the punishment for revolting, and the answer was death. So faced with the same punishment if they revolted or late for work and they were already late for work, they decided to revolt instead.
While the current punishment for piracy isn't death, it's the same if you pirate a movie that would of been in the public domain vs. something that is current and wouldn't be in the public domain; then to the pirate, what is the difference?
Of course to the pirate, nothing, but if the purpose of the piracy is civil disobedience (which the OP was talking about and you responding to) then it does matter because it would be to express the idea that Copyright is broken.