I think this highlights several important issues. You wrote a message on the bag expecting/hoping(?) to get a rise out of someone. This wasn't done in an effort to educate but to ruffle feathers. Trying to educate the TSA screeners (if such is possible) in the brief interaction you have with them about how their jobs aren't contributing to airline safety is like trying to tell a smoker who isn't ready to quit.

Would "You should be screening air cargo, not my toothpaste" have gotten the same response? With that particular set of screener/supervisor, perhaps. Would it be more educational than a "Kip Hawley is an idiot" message? Most likely. But would the idiot message get a rise out of someone? Clearly it did.

So inviting trouble, then complaining about the trouble you get for your efforts seems like a waste of effort. The screener's comments about your rights were inappropriate but you were clearly hoping for that type of confrontation.

In the end though everything worked out. The LEO made sure you weren't wanted for anything, you were detained for a brief period of time and got to make your point. Like the guy that tried to board a plane with a "We will not be silent" message in Arabic, you invited trouble and got it. Fortunately unlike Raed Jarrar it didn't escalate so dramatically.

Personally I feel his shirt was completely appropriate, ignoring the wearer's supposed history of trying to stir up trouble. But whenever you try to draw attention, be prepared for the consequences.