I think you actually mean accepting cash as a valid payment method, and having card only TVMs gives people a valid reason to pay on board, which is their first opportunity.
Yes, it does give people a valid reason to pay on board, but my point was more along the lines of it makes it easier for a fare dodger as well, which we don't want.
Consider someone who has no intention of paying until challenged, but is also aware of the penalties (including the risk of prosecution) and will try to avoid them. If they have a card on them with the funds, they only need carry a few quid in cash to cover the fare as well to have an excuse of not being able to use the machine. This also means that if, as can sometimes happen in FGW land, they don't see a conductor on board, they need only walk out at their end station to avoid a fare.
If there was a revenue block at their end station.the only thing they'd have to do is pay the relevant fare alongside law-abiding passengers who genuinely intended to pay for their ticket. However, if the rules meant they HAD to pay if they had the relevant form of payment that the machine would accept, they'd be bang to rights and have to be written up whilst the law-abiding passengers would be allowed to buy as normal!
As an aside, if the railway only accepted cash and someone turned up with not enough cash (or some cash, but didn't want to use it) they'd be treated as a "can't pay won't pay" passenger and dealt with appropriately. Allowing someone to refuse to pay with a certain type of payment when they have the ability to seems, to me at least, just another opportunity for some to play the system.
