I don't think anyone would seriously argue that public figures should stop speaking in public at all because someone might faint. On the other hand, when the person directly behind him fainted might have been a good place to stop immediately, rather than carry on for another few minutes.
There's also the concern that, while, yes, the campaign has probably started in practice, in theory it has to wait until Parliament has agreed that there should be an election. That hasn't happened yet; until then, it could be argued to be more than a little abusive of the privileges of office to hold what is effectively a campaign speech with a row of police cadets behind you, who you presumably took out of their normal duties.