Maybe because of a Terrorist attack like 9/11.
A terrorist can hijack a plane and fly it into a stadium killing thousands so its a necessary precautions
So what? If there SAM's on top of rooftops when 9/11 happened, it could have shot down those planes before it hit those sky scrapers.
So its a necessary precaution.
US does not need SAM's on rooftops in New York because it has deployed Submarines near New York to shoot down any plane which is threatening the city.
Since You can't deploy Submarines in the Thames River and a Terrorist attacked occurred in London carried out by Islamic Terrorists on July 7, 2005 and London has thousands of wannabe jihadists who look at the London Olympics as a golden opportunity to launch a terrorist strike, so announcing SAM's on rooftops is just as a big deterrant as having SAM's on rooftops.
A terrorist will think twice about hijacking a plane and flying it into a stadium when a SAM will shoot it down before it gets to its target.