Probably because Stephanopoulos got a hurried signal from a program director that Ross had some new information. In the chaotic nature of a breaking story, I suspect Stephanapoulos did not even know what Ross was going to say. I think this one is all on Ross.
What if the "hurried signal" from the producer allowed Ross to accuse the shooter of planning an assassination attempt? Would they have speculated about it and continued with more rumors with that as a starting point? Of course they would have. They are ALL responsible for any misinformation that comes over their airwaves.