That recommendation is more so that people will have enough range. There seemed to be a lot of these cultural myths in society among people who don't understand electric cars that leaving an electric car plugged in too long would overcharge it and blow up the battery or something. Tesla is just trying to overcome that fear by letting people know that there is no reason to avoid plugging in, and it's helpful for you to just plug it in as often as you want, so you don't find yourself without enough range to drive. That would spawn complaints of electric cars being deficient, etc., which is negative PR they don't want. Anyway, you mention 4 or 5 days. It'll be fine and not a big deal. You will probably lose somewhere around 1-2% per day, but if you had some decent amount of charge when you get there, it won't be any issue sitting for that long. I mean, think about it: the cars sit unplugged for longer than that just being transported across the country.
Why are you sure of this? Whether it is against etiquette or not is very simply answered by whether this is in a long term or short term parking area of the airport. If it's in a short term area, then sure, those are meant to be used for an hour or two and then leave, but people don't use long term parking for two hours. The whole point of the existence of long term parking is to leave your car sitting there for multiple days, so if there are any charging stations in there, the expected use it to leave your car plugged into them for several days while you are gone. Here is a thread with a whole argument on this about the Denver airport parking areas and charging stations.
Denver International Electric Parking - Teslas Stop Parking There!