It may depend on whether there are any fares priced with the walking interchanges in mind. Ware to Bayford is only available via London, meaning that if you wanted to make the journey via Hertford you’d need two tickets, or two PAYG fares. Ware to Stevenage is available via Hertford, but it may depend on whether the fare is different to the sum of the two singles. The same consideration will apply to other pairs. If the train companies want through fares to be available using PAYG and those interchanges, they’ll have to ask TfL to set up new OSIs.