Have you ever ran a Marathon & do you have the 411 on it's regulations/guidelines ?

The Boston Marathon is being held today,I always thought marathons were open to the public as long as you were willing to run in the back & space was available but this site states the only way to run is to qualify by running another marathon within a specified time or to raise a +3k donation ? This year the race was limited to 25,000 entries,I cannot believe they are all well conditioned athletes or people willing to donate $3,000.
Also do they have different starting waves for the elite runners, seriously trying win the race and/or set a new world record ?