Hisense warranty voided

I purchased a 50 inch Hisense television from Hifi Corporation. After a month the television had power but no picture or sound. I took the television back to Hifi Corporation and they sent it in to Hisense for repair or straight return. When i went to pick up television the Hifi Corporation staff let me know that the warranty was voided because i tampered with the television.

All i did was i changed the 2 pin plug to a 3 pin plug. The power cable is not even attached to the television set. It came in a plastic bag with the remote. So how is that tampering with the television. The cable is not even a specialized part. The service manager of Hisense even said i should have just bought a cable with the 3pin plug already attached and it wouldn't have changed the warranty. That in my understanding of common sense makes the cable an accesory.

I have even opened a case with the CGSO this is how dumb this whole situation is.