I'm sure their parents have equally expensive road bikes and don't lock them up either in similar contexts.

I have been road riding for eight years now, often with clubs where we leave a bunch of expensive bikes out without locking them up, and am yet to hear even a rumor about someone getting a bike stolen from such a pile up. Of course, we always keep an eye on them, but it doesn't seem like that's how bike thievery operates.

Interestingly, the riders of the club I ride with now - attracting an older, less athletic demographic - tend to lock their bikes up when we stop for lunch. However those are long stops where we are inside and the bikes are out of our sight. When we just hang out around a coffee shop, we don't lock them up either...