That's merely an assumption. The wording is that the machine would not accept cash, not that you thought it wouldn't accept cash. I'm probably just being pedantic.
If a machine does not have any slot into which cash will be inserted, then it categorically does not accept cash. It is not an assumption, it is fact. You don't have to use it to see that it neither has a coin nor note slot and thus cannot accept cash, you just have to look at it.
And judging by the trends in the industry away from, not towards, cash, if it didn't accept cash yesterday because it didn't have the requisite slots it probably won't today either (though that is an assumption).
In reality, I'd expect anyone with cash would be sold a normal ticket, not a £10 special.
The PTP is mostly a modern version of a Permit to Travel. Thus if the passenger asks for a ticket from the previous station when it's patently obvious that that's not where they boarded, at that point it would be requested and £10 charged.
Indeed it's in part a guard against short faring - no point doing so if it's going to be £10 anyway!
It's quite clever in quite a few ways to be honest - it deals with the most common forms of short distance fare evasion without being so expensive that you wouldn't just take it on the chin if you were not a fare evader but were in a massive rush. Basically exactly the same as the tram, but with the option for a longer distance fare to go above £10 because the railway has plenty of fares above that whereas the tram doesn't.
A
very small number of MoneySavingExpert types (often found on here) might legally challenge it if charged because they don't think it's legal (though to be honest I do think it is legal). Even if successful, "neds" at whom this is aimed won't. They are barely literate enough to even write a strongly worded letter of complaint. They will either cough up £10, be put off by the risk and so not go by train (not likely they will pay) or just do as before, not pay and run away when challenged. And people who are a bit more literate but short faring will be caught, cough up once and pack it in.
It will work.
The only real edge case is someone who only has cash and chooses not to print off a Promise to Pay. In reality, Scotrail's TEs tend to be sensible folks; the lack of a Promise to Pay probably wouldn't make them charge a tenner cash, but presenting a card to pay probably would.
Or blatently short faring.