I strongly disagree. If someone boards a train and is prepared to pay on board then that is, in no way, an attempt to fare evade. The courts would laugh at such a suggestion! I have done that myself, a good reason is if I am taking a bike and I don't know if I will get it on. Why the hell should I pay for a ticket I may be unable to use if I can't get it on? The guards were happy to sell the tickets and had a completely different attitude than you did (yours is libellous - nothing less). People who buy on board do the TOCs a favour as then the TOC they travel with gets 100% of the revenue, and they may even get commission for the guard too.
I recall a few years ago, when waiting to collect my bike from a DVT (so the rear of FC), a (obviously wealthy) passenger saying how she always buys her £200+ FORs from the guard as she wants the guards to get as much commission as possible (rather than it just going to the company) as that's £10 for the guard, the person she was talking to was either a guard or a customer service assistant, not sure which, but was appreciative.
As for whether it is a "punishment" that you are sold an Anytime, that's certainly not a legal view. If that is the ticket you would have bought anyway then why not buy on board? If there is a small price difference, then you may accept that for convenience. If there is a large price difference then only a fool would do that, but if they make that choice than that's their choice to make and if they want to pay a higher price, then let them but don't accuse them of evading if they ask for a ticket!
The "customer is always wrong" Basil Fawlty-style attitude isn't by any means among all or even most rail staff, but the stinking attitude is among a significant minority, and it needs to be wiped out. It doesn't exist in any other service industry (except in very few cases which would get that person disciplined). Why does it exist in the rail industry? It's a problem that the rail industry has, and it needs to stop, because it stinks.
There are many stories here of people treated badly by the rail industry. Why is that? it needs to stop, but it will only stop if the poor attitudes among some of it's employees changes. people who have that attitude do not in any way promote train travel and put their own jobs and the jobs of their colleagues in danger, in my opinion.