Word sounds totally logical that an off peak ticket could not be used during the two peaks when the majority working and not working wish to travel. When commuting I see plenty of people in education, shoppers and wot not who stump up the extra for the convenice of travelling at peak hours. But I know plenty of workers who work later or stay up town to get a cheaper rate. Logically any operating compay to discourage those who have a choice when to travel would give an incentive to travel during the off peaks. Also logically the laws of supply and demand say if a commodity is in demand then the price of said commodity rises
Many people are happy to work 9 to 5 mon to fri and often are happier to settle for less pay/benefits than they might get if they worked unsociable hours. My Wife has just moved Jobs and although now a Manager gets slightly less generous benefits but now works more sociable hours so is delighted
I see and know of people who work late or early in Jobs and some Moan as it hinders their social or family life but do it as they have to, others find it suits them better horses for courses and all that
Lets face it you paid less so could have spent the saving on a extra book or a drink people watching at the station the world go by or reading
By the way I found from experience different operating companies have different policys so best to check the rules for each one some except off peak tickets in the evening "rush hour" others dont. One thing im a bit confused you mention you bought a off peak travel card, the only time travel is restricted is before 9.30 mon to fri. However the oyster card has two peak rates a day (check the TFL site for details)