You don't seriously for a minute think Kim makes these calls herself do you? Her agent, or more likely her agent's administrative assistant, makes the calls/send the emails. She might on occasion buy something like a plane ticket herself but she no doubt doesn't even have the contact information for the sponsors at the race track (the appearance was for an advertiser, not for the track/races themselves). And given the massive time difference between Australia and the US (where Kim's staff probably was located) I think a few hours to communicate a change is reasonable.
I'm not defending KK here, honest, I feel like there are many legitimate things to think she does that are not tactful, but this is not necessarily one of them. She cancelled an appearance. Its unfortunate it was picked up by the media before 'she' (meaning her or her staff) could contact the appearance people, but if you google for the story, 'she' did in fact send an email Thursday (her appearance was scheduled for Saturday) - she didn't just no show, relying on the media to inform her employers.