Why do Oakhurst and the others avoid telling the young couple the truth about why they were not in Poker Flat?

"They'll find out the truth about us all when they find out anything," he added, significantly, "and there's no good frightening them now."

John Oakhurst tells Mother Shipton and Duchess to keep their expulsion from Poker Flat quiet in order to not scare Piney and Tom Simson.  Tom is also known as "the innocent." The name is a solid fit, because he can't even recognize that Duchess and Mother Shipton are prostitutes.  In fact, Tom...

"They'll find out the truth about us all when they find out anything," he added, significantly, "and there's no good frightening them now."

John Oakhurst tells Mother Shipton and Duchess to keep their expulsion from Poker Flat quiet in order to not scare Piney and Tom Simson.  Tom is also known as "the innocent." The name is a solid fit, because he can't even recognize that Duchess and Mother Shipton are prostitutes.  In fact, Tom thinks that Duchess is John Oakhurst's wife.  

When John asks the two women to keep their expulsion a secret, it is after Uncle Billy has stolen the horses and everybody has been snowed in.  Oakhurst knows the situation is not good and will quickly turn dire if immediate actions are not taken.  By not telling Tom and Piney the truth, Oakhurst is trying to keep them calm.  Oakhurst does not want Tom and Piney panicking over the fact that they are snowed in with a bunch of moral degenerates that have been cast out of a nearby town.