I think there are similar constructions, perhaps less reputable than "blame" The travel agent isn't to blame. (Don't blame the travel agent) This food isn't to eat. (Don't eat this food) This car isn't to drive. (Don't drive this car)

It means that the travel agent wasn’t responsible for (wasn’t the cause of) the overbooking. It’s like saying “the travel agent isn’t the one who can be blamed for the overbooking” or “the travel agent isn’t the one to bear the blame for the overbooking.” But today we now use a shorter version like you saw “the travel agent isn’t to blame for the overbooking.”

Yes, it's an odd grammatical construction. It means: the problem was not the travel agent's fault, or more literally: "the travel agent was not (the person) to (be) balme(ed)".

To say someone "isn't to blame" means that they should not be blamed for what has occurred. In other words, it isn't the travel agent's fault that the overbooking occurred. This is a special construction, but it is only used with "to blame." I cannot think of any other verbs that can be used in this construction.