The answer is that people are human and they usually realise that they are ruining something for other people above and beyond their self-involved act. Like when you go to a concert and somebody puts their girlfriend/child on their shoulders for a vast period of time and then the people behind them can't see anything. That person usually gets the message that the people behind can't see (or not, and then some irate person throws a bottle of something-gross at their head - we've all been to "that" concert when "that" happen!
I realise 100% that 114 is a supporters section - I realise that people can bring in banners/flags/drums etc. I've been a SSH since day 1 in the south end so I've seen it all in the south end. But the line is drawn at where you can't see vast areas of the football pitch for vast parts of the game. If the thoery that "you should expect to see flags" is true to the point where you can't see the play, then I will bring in a 6 foot tall- 6 foot wide banner to the next game and hold it up continuously throughout the game - however I'm almost certain I will be asked to lower it by the 5th minute of play when the 100 people behind me can't see anything at all.
There is a massive different "imo" between expecting to see some flags/banners during some parts of the game and being behind a flag so large that you can't see anything on the field for vast portions of the game.