If you want to know what they can legally do, read your warranty book in the glove box. On page 14 it says tampering with the odometer system WILL void the (entire) warranty. Page 17 says tampering with the emissions system will void the emissions warranty. Page 18 says they can restrict the warranty, and that is what they apparently have done. If you want to know what to do now, see page 37 and beyond.
Before you go make a big deal out of it, remember you violated a federal law with the cat removal. If I were you, I would contact DC and explain calmly your side. I believe they will agree with you, but most likely they will leave the restriction (or partial restriction) in place so they will have to be notified BEFORE any repairs are authorized. If they think you might have compromised the odometer system, you are out of luck period. Keep in mind odometer tampering is also a federal offense. You will also have to sign a federal odometer statement saying the mileage may not be accurate. Also if you sell it to a dealer and don't tell them, they (and then you) will be sued if it is not disclosed. So again, I would be very polite in what ever you do. And remember this is a public forum where you admitted the tampering for all to see.
Not arguing the decision, just trying to cut through the clutter to the facts.