They are checking regularly, which is why they're finding the abuse. It's customers not activating tickets until the conductor pops along in the hope that they don't have to activate one for the day.
That's got absolutely nothing to do with the ticket type though, has it? It's no different to "pay when challenged" (or nowadays, "buy when challenged"). mTickets that haven't been activated prior to boarding aren't valid, simple as that.
On metro lines with frequent stops like on the routes in the Valleys, it's never going to be possible to get everyone.
It'd be a lot easier if guards didn't have responsibility for the doors... There are plenty of tram systems that operate on a "buy on board" basis (e.g. Sheffield, Blackpool and Birmingham, no doubt others abroad too) with even more frequent stops, so I simply don't buy this as an excuse for making life harder for law-abiding passengers.
You either gate every station (norm practical) or you accept the product isn't practical because, sadly, a sizeable, significant minority of your customer base can't be trusted.
Or you accept that a small percentage of people will inevitably get away with not paying the fare, but that you can keep the majority honest by conducting regular checks. Again, this has nothing to do with the ticket type - presenting a MultiFlex that was activated after boarding is no better than offering to buy onboard.
I do think people like you fail to understand that fraud/misuse isn't just one or two people per train, it's become an emergency - ordinarily good, honest people have turned to fraud because they can't sustain their lifestyle/commute/whatever.
Thanks for the generalisation, but I'm perfectly aware of the amount of irregular travel that goes on. By all means, punish the guilty - but that's still not an excuse for inconveniencing the law-abiding majority. This attitude is like being at school again - "own up or we'll punish all of you"!
This is compounded by whenever the railway cracks down on this, they can't say that the people running to the press are downright dishonest
Fraud requires there to be a false representation. I see no such representation here.
If there really is evidence of fraud, the train companies should feel free to say so. If there isn't, they shouldn't be throwing around such terms carelessly.
nor can they go into details of the fraud for fear of making it even more known and abused.
I'm afraid this is just an extension of the "security by obscurity" mindset, which is a complete non-starter.