Why would they look silly, when the validity of the ticket is clearly advertised?
(I agree it should be changed, but would people pay a corresponding increase in price - 20-25%?
Because the name is highly misleading if the validity is so very, very different to what the name implies!
If they called it a Southern Central Day Ranger, for instance, (though of course that's not nearly as catchy a name!) then it would be more understandable. But there is a fundamental implication if you get a ranger named the "Oxfordshire" Day Ranger that, I don't know, it's valid throughout all of Oxfordshire!
If the ticket office staff didn't refer to any terms and conditions either when selling the ticket (e.g. not saying "the map of validity is shown online, please check it", "are you aware of where this ticket is valid?" or "you're aware this ticket isn't valid towards Bicester?") then I am minded to believe that there could be a potential legal case for misrepresentation if any train company took umbrage to the use of the ticket in a way that the name would imply is permissible, but which the train companies don't actually want to permit. Of course, that's all rather theoretical. At the end of the day, they either need to change the validity of the ticket or change the name. But the current situation is not really palatable.
I don't think they would need to increase the price, from the perspective that the Ranger is currently priced at just more than what I think is the most expensive Off-Peak Day Return that is covered by it - Moreton-in-Marsh to Reading. That's £16.80 and the Ranger is £17.50. I can't find any Off-Peak Day Return to or from the Oxford-Bicester Village branch that would be more expensive than £16.80, so there wouldn't be any revenue risk as such.
That's, of course, not to say that the price wouldn't be increased to reflect the additional validity, but it would be rather unfair (in a way) for this to happen, given that the branch
used to be included!