Well I can't speak for the individuals that are posting their food diaries, but the point of a food diary is really to detail what you are eating so that you can see if you need to up intake of a certain group, or find a reason as to why you aren't losing the way you would like to. If we all ate perfectly all the time, there would be no need to keep a diary to monitor it.
Remember, these are real people writing down what they really are eating. It is important for them to do that, for their own benefit in terms of seeing how well they are sticking to the plan, and how much of an effect that has on them. But its not really something we should stand in judgement of, because as I said, not everything will necessarily be detailed down to the last ingredient.
We are given the plan, and in an ideal world, we would 100% stick to the plan. But there will always be days, or even sometimes weeks, where its not so easy to do that, but it doesnt mean we shouldnt keep a record of it, because while it would be very easy to fictionalise our food diaries and make them look as though they fit the plan, we are only fooling ourselves if we do that.