Well, to try and answer your question, an experienced investigator, with any common sense at all would look at the entire picture. He/she wouldn't say, " One fish too many, get the pinch book out".
Here you have two non-resident anglers, who were cooking their 4 fish. They were within the "spirit" of the law. One, when asked the question, readily admitted that he caught three. For the sake of argument, as this info is not known, they both had the appropriate licences. There was no attempt to hide the fact of who caught what. They were honest and forthcoming. It is not a stretch to believe they did not know the party fishing rule, and in all likelyhood, articulated that to the CO. It would have been in order for the investigator to come to that conclusion.
Had they had 5 or more, then clearly they were in violation. The fact that they were within their legal possession limit, notwithstanding the party fishing rule, should tend to make an investigator believe they had no "intent" to violate their possession limits.
If you can't understand this concept of enforcement, I can't help you !!
To edit: You have heard terms like "spirit of the law", the "intent" and "the rule of law", every LEO understands very quickly the meaning of these terms where enforcement is concerned. The courts at trial also consider the same aspects of the legislation. A lot of new LEOs that subscribe to your "charge them all" mentality have had their arses handed to them in open court. That only serves to bring their credibility and judgment under scrutiny by the JP or Justice, whatever the case may be. As I said in an earlier post, a LEO that conducts himself or herself, with your mentality will not last long.