I thought I'd be going off at a tangent here but the conversation seems to have drifted towards customer service anyway. I hope you won't mind my observations on why people on the wrong end of a penalty fare think that the guard is "rude"; I'm sure this is mostly well known and common sense, though.
People in this country (and, I suspect, many other Western countries) are conditioned to expect a very conciliatory attitude from front-line customer service professionals. It's not the oft-quoted "the customer is always right" - as a lot of the time they're not! - but in most cases, the person serving you is expected to be friendly and helpful, and when they have to give you bad news they're always apologetic and eager to try and mitigate any loss or damage caused to you.
Consequently, the uncompromising attitude (friendly or not) of RPIs and train guards throws these people for a loop. The guards and RPIs among you know this kind of response very well:
How dare they treat me like a criminal! Who do they think they are, the police? They're just some jobsworth, probably on commission - I just made an honest mistake, why are they being so rude?
And in a split-second, the customer's image of the staff member changes from an ordinary front-line customer service professional just doing their job, to a hard-nosed authoritarian figure there to deliver stiff punishments with no remorse. It's a (perceived) attitude that people only expect from high authority figures, like the police or court judges, and it rankles them when someone who's "just doing a job" has the audacity to treat them like that. It doesn't matter whether or not you think you've been rude - to them, the fact that you've deviated from the normal customer service "pattern" means that you are rude, and they'll match your rudeness with their anger.