In short, they have nothing to lose by being truthful.
In fact, they have a great deal to gain by telling the truth. Apologizing publicly to Eunice Stone for jerking her chain, scaring her out of her wits, and causing an alert in two states will go a long way to healing wounds right now. And further demonstrate that they are responsible, clear thinking and contrite young men who intended no harm.
The majority of Americans don't care if they are Arab or muslim, just that they be honest.
Have you considered the possibility that they are telling the truth?