Do they have day jobs for the summer? Are they fulfilling any duties, responsibilities etc. that they have or what you ask them to do? If they're generally responsible and good kids, personally I wouldn't worry about their internet use or when they use it. Sounds like they are pretty smart kids.
Saying that, if they are irresponsible for not obeying your direction as a parent, that would require consequences. Guess you need to decide.
We are told by our government NOT to judge Muslims by the actions of a few crazies. We are also told by this same government TO judge ALL gun owners by the actions of a few crazies.