This thread asks a simple and pertinent question politely and reasonably. That's not trolling behavior. Let's be equally polite and measured with responses.
FWIW, I agree that this is JetBlue's decision to make. I could understand their going either way, but as in other areas, the decision needs to be consistently applied once it is made. Inconsistency will drive customers much more crazy than will a policy they disagree with.
Since I'm pretty clearly in the minority of posters here I'll consider my question answered. Thanks, everyone.
For the record, here's what would have made this an experience much more typical of the excellent history I've had with JB. A small sign/label/etching on the seatback in front of each EML seat that says:
"Jetting loves legroom. Welcome to one of our extra legroom seats. There's a small fee for all this extra space. If you didn't pay for the seat when you booked, our friendly flight attendant will be happy to swipe your credit card for you on board."
A sign like that not only reinforces the desired "elite" status of the seat, but makes any subsequent conversation an easy one. Point to the sign, ask for the credit card.