sir82: I dunno, I wouldn't necessarily find such an announcement "shocking".
I've attended conventions in urban areas where there were lots of idlers hanging about outside. Once they figure out that the arena is completely open, has air conditioning, and there are people who tend to leave stuff on their seats unattended for hours at a time....seems like word would spread amongst themselves that there is a lot of "potential" there.
I don't think she had an issue with advising the audience to secure their belongings because of the possibility of theft. I think the problem was the blanket insinuation that persons who are not delegates are automatically untrustworthy while the delegates were all above suspicion. That's an unfair and offensive prejudice. Most people are not thieves. A minority of JWs are thieves and a minority of non-JWs are thieves. Maybe the percentage of non-JWs who are thieves is higher than the percentage of JWs that are. But in both cases I'm willing to bet that it's a small percentage - a small minority - who are thieves. So it's highly improper to negatively label an entire group of people because of the bad actions of a few individuals among them.