Having worked in various joint commands in plans (J5), exercise (J7), and operations shops (J3), I have to tell you these things are FAR from free. Military exercises, deployments, employments, redeployment, and reconstitution (after the fact) are quite expensive, in fact. Literally hundreds of millions of your tax dollars are spent on these each year to maintain unit readiness.
Difficult to stage? yes... Scarcity of opportunity? no. Quite the opposite. Military commands at all levels routinely have to cut exercises and practices from their schedules to support various "hobby horses" or "pet projects" the military is tasked to support based on political pressure. The National Jamboree is a good example of such a "pet project."
Do the units supporting these get good training? Absolutely.
Could the resources spent on the Jamboree be better used supporting other valid military training objectives? Absolutely.
Do I support the use of military resources to enable the Jamboree? Absolutely 😜 (Sometimes the troops would rather support something at home like this, rather than flying to a third world country to practice their "wartime" skills there.)