The issue with 'short' tickets on railtours, is that passengers who might have previously done the full tour may be tempted to save money by only doing one leg or a section of one leg.
This is where it comes down to the experience of the tour organisers in knowing when to charge part fares and when not to - and also charging a suitable amount. The example given elsewhere where someone was quoted the full fare for doing the rare track on a tour (ie the main part which everyone else has paid full fare for) is absolutely correct in my view - had he been charged half the fare everyone else who had booked in advance had paid, they would have understandably been somewhat aggrieved and may well decide to do the same themselves next time. The end result could be a tour does not get enough advance bookings so does not run as it is not commercially viable. A balance has to be struck.
I've had experience on both sides of the fence, as a 'gripper' for one tour promoter and as a punter with others. In many cases, people will have a good reason for doing a part fare - for example I was away on holiday in Devon when Spitfire ran a Crewe-Paignton trip with a pair of 37s, one of which I required. It was impossible for me to do the entire tour so I did a part fare out of Paignton (as did a fair number of people who actually lived in the West Country) and I'm sure everyone appreciated it as I did.
Similarly, on Retro's trip to London (West Hampstead) and Southend earlier this year, a part fare was offered between West Hampstead and Southend. This was a sensible thing to do, as there was space due to a good number of people alighting for London. This enabled people living in the south to travel on the tour and have some Class 31 haulage - for the majority of them, doing the full tour would not have been feasible without hotels/long journeys.
As someone mentioned, not all promoters will offer part fares, the BLS being one. That is, of course, their right and something people have to accept. After all, you wouldn't go into WH Smiths and insist on being able to buy part of a book or magazine!