Health and Safety regulations are not restricted to people directly involved to an event or site, responsibility applies to people in the surrounding area. This is why those running the fireworks event associated with the M5 crash were brought to trial for HS breaches, although the case was thrown out due lack of evidence.
In principle of law, event organisers could be held responsible for what happens to people outside the designated display area. Guilt would hinge on whether risk could be "reasonably expected".