In addition to the signage issue in the Vail case, it also appears they failed to do avalanche control work in that area when they clearly should have and typically do. It will all come out in court, but it seems something broke down in the patrol process where they believed they had done control work but hadn't, IIRC.
Is that fact or speculation on your part. If fact, where did you see it. As far as failing to do avalanche control work where they should have--it is common after a snowfall for full avalanche control work to take several days and a run may remain closed because patrol was not able to bomb and cut it in the time between first light and opening. In that situation patrol may delay lift opening to continue control or leave the run closed for the day and resume control work the next day. So the fact that the prima cornice wasn't bombed seems to be irrelevant--since it was closed. If the lower gate was open because someone thought the upper run had been bombed and it hadn't been, that might make a case, as would the demonstration that Vail deviated from its formal protocols, if it in fact did, in my entirely uneducated opinion.
People seem to be thinking that the case hinges on whether patrol should have anticipated people hiking up from the lower gate and prevented that by sign or rope closure. It seems to me that patrol has another defense, however, which would be that they felt that hiking above the open gate was safe, as long as no one entered through the upper gate into the potential start zone of an avalanche. Something we see frequently at Alpine Meadows is Low Beaver entrance open with High Beaver closed. The High Beaver entrance is much steeper, more avalanche prone terrain directly above the low beaver terrain, but is felt to be safe to ski under as long as no one enters from up there. Obviously, that would apply in situations where the risk is moderate--natural avalanches unlikely, human triggered avalanches possible--or low. I don't know Vail so I don't know that my theory would apply to the Prima Cornice slide. Obviously, if that thinking was the case, Vail Patrol got it wrong--but just as avalanches are an inherent risk of skiing, patrol misjudging avalanche risk is an inherent risk; it is far from exact science.
The rarity of death and serious injury from inbounds slides in the face of frequent deaths in the backcountry demonstrates that patrols do a good job in mitigating the risk. The fact that they occur at all and at a number of different areas demonstrates the imperfectability of mitigation.