Thats pretty much it Chris - basically you are paying for the 'free' travel by putting yourself it a position of responsibility.
If you don't ID yourself and pay, then you could just sit there and do nothing, you have to risk assess your involvement with the nature of the offence - obviously the more serious the offence there is more obligation on you to act - and indeed a failure to act in some circumstances can be considered a deriliction of your public duty, which can lead to discipline or even a charge in extreme cases.
However there is no point in intervening if by doing so puts you or others at risk of serious harm, or is unlikely to help matters at that immediate time - in those circumstances you will need to take a more subtle appraoch - descretely calling for help, noting descriptions, obtaining witnesses, following until help arrives etc
It ain't all black and white