This actually feels like a "well we have always done it like that" situation. Surely when you operate one train a day on a route you want to make it affordable.
Avanti can make the one train a day affordable with Advance tickets.
The reason for it will be historical and will be related to the fact that there is only one or two trains a day to Blackpool. The purpose of the restriction is to keep the peak fares higher for Preston and points south. If Blackpool (or Bamber Bridge, Blackburn, etc) had the same fare restrictions as Lancaster instead of Preston then it would be a lot harder to enforce, as nearly everyone using the ticket would be changing trains at Preston.
So essentially there is no issue, and you can in fact change at Preston on the outward journey? What it says upthread is that for whatever reason this is not possible.
Nearly all the Avanti trains from London go on to call at Lancaster, which is about 15 minutes north of Preston.
Therefore, if you get off an Avanti train at Preston to "change trains" or "use the station facilities" before going on to Lancaster then any revenue protection staff will- fairly understandably- give you something of a grilling as to why you did that. It isn't that you
can't do it.
But almost nobody genuinely travelling from London to Lancaster would get off a train at Preston unless that train doesn't call at Lancaster. Nearly every Avanti train calls at Lancaster. So anyone who does do it will need to provide an explanation as it is an unusual thing to do.
The enforcement tends to be more about the southbound journey, though. You can't enter the station at Preston with a Lancaster to London off-peak return. That's easy to enforce.