I believe the rule about having a parent/guardian is specifically for athletes on the National Team attending camps and competitions.
While it may not apply to JO programs, I imagine that as a safe-guard most gyms would adopt a similar policy.
Our higher level teams are just now starting to travel to meets more than just one state away and all the girls have a parent they are traveling with. As a coach, I would not want to be responsible for traveling on my own with multiple child athletes.
Yes, college athletes travel together - but they are already away at school, and travel with multiple adults (coach, asst coach, athletic trainers, etc.)
When I competed we would travel as a team to one meet a year, but pretty much everyone had a parent with them. We (the gymnasts) roomed with our teammates, but our parents were in the same hotel and usually the night before the meet we stayed with our parents.