They do understand but they're also in their early twenties and privileged beyond belief. The reality is that the York girls, and to an extent the Wales, know they can do as they please without much consequence.
The "immortality" someone that age feels makes for crap decision making. Add their status and you're going to get a ton of mistakes and constant bristling at the attention paid to their actions. Fifty years ago, hell thirty years ago it would've been unimaginable but today, especially with the older generation still churning out the duties at such a blistering pace, these kids don't feel the pressure.
The Yorks will continue their calm existence for as long as they can and then they'll settle into duties as they're passed along to them. If anything the media coverage of today's world means that, despite being the children of the second son, they will have a lot more clout in the future than previous minor royals but for now. Meh.