I don't have any similar experiences to share, but for what it's worth, I agree with you that the official reaction to your girlfriend's babysitting is unbelievable, and IMO shocking.
I appreciate the fact that being bounced is never fun and it is unfortunate, but to say that being punished for breaking the rules is "shocking" is pushing it, in my opinion.
Please understand what this looks like from the ECO's point of view. You lied about being bf/gf. And it wasn't an accidental lie, it was clearly planned because when your girlfriend said she was a friend, you backed her up. Furthermore, she accepted compensation for doing work, not a trivial violation by any means. They stamp your passport with a warning about working for a reason. Ignorance is never an excuse for breaking the law, especially one that is fairly straight-forward.
Now, you might wonder why I am even saying this to you and partially, because it is annoying to people who do their research and follow the rules and make an effort not to lie to ECOs to read about you complaining about how unfair this whole system is while in reality is happens to be one of the less strict ones out there.
But partially because now that you've pretty much hosed your chances at a visitor visa, for your gf to come to the UK, you'll need to resort to the family formation scheme or possible the Tier 1/2 visas if you qualify and to get those you need to put aside your thoughts on the unfairness or lack thereof of UK immigration, and follow the procedures to the letter!
You could have avoided the problem you're now facing by having done your research. There is advice without number out there about NOT engaging in the gf/bf or only a friend word games and also there are specific warnings about NOT bringing diaries as they cause nothing but grief at the border. Not to mention, both of you would have known about the hassles American women now routinely get on arrival here.
Take the above not as a scold but as a sign that whichever path you'll decide to go in the future, do very thorough research and follow the rules, putting aside their fairness or lack thereof and hopefully this will be just a hiccup in your past.
And take garry's advice and get the SAR. Assuming you'll decide to apply for any visa at all, the person deciding your case will be looking at that. You might as well know upfront what he/she will be looking at.
And good luck!
ETA: to avoid confusion, I'm addressing this post to the OP, not historienne. I simply wanted to specifically respond to the phrase calling the application of the rules "shocking."