Where to Daven in a Shul

After reading Dvar Phutims post here it got me started on another thing that drives me crazy which is something I will never understand no matter how many people try to explain or rationalize it. It is clear from halacha as seen in Mishna Berurah when one goes to shul they should not sit by the door or stand by the door because it gives the impression that you are rushing to leave shul. With that in mind I have a question. Whenever I show up late for mincha someplace I can never get in. There is this phenomenom that people like crowding by the door when they are late and are scared to go in. I understand that if someone is davening you cannot go within 3 amos of them but if they are in a position that is blocking you, i see no problem with walking within their three amos to get by. By people blocking the entrance they are preventing other people from coming in to daven. To me this looks like selfishness. Why can't you make your way further inside. The minyan I go to, the Rav stops and tells people to move to the front. When people are gathered by the door or in the back, it also leads to talking. Preventing someone from getting into shul is really bothering to me and Rabbaim need to place signs on doors stating this so it doesn't happen.