Why does this offend you so much? Most people consider AR-15s Assault rifles right or wrong. I suppose we should be glad he didn't have the full auto military version but I really don't get your outrage.
Blaming the news for reporting that he used an Assault rifle when most people likely agree the AR-15 falls within that common definition makes no sense. It's frankly unbelievable that you are more bothered out the term "Assault rifle" being mentioned vs the actual shooting. I'm sure the family of the murdered will be comforted by the fact it wasn't an "Assault" rifle and "only" an AR-15 that killed him.