Erm....that is why they are deployed.....it is a little concept called Deterrent. I know, let us have no security at all, advertise that fact and see what happens.....
Yeah, because we should stop living our lives simply because of potential threats......most people will not even know they are there.
It's not a deterrant, having sam sites on roofs to having more difficult security measures at airports and generally less freedom... this is the actual goal of terrorism. They don't like they way we live, they don't like us living in freedom, loss of life is disgusting and utterly evil, but its not the end goal.
Every time we spend more money on security and make our lives a little less free, they win.
Every time we react to a terrorist attack with changes to the way we live, they win, them seeing us change encourages them, it just shows they can effect the way we all live our lives.
Anyone, anywhere can kill one person you can't plan it, nor stop it effecting every last person in a nation of 250mil is a much harder thing, the USA's reaction to terrorism has effected far more people than the terrorist act itself.
If they did plan and carry out an attack at the olympics, and that attack is foiled by a sam site shooting down a plane that was due to crash into a stadium, when we then decide its a great idea to stick 1000 of these around the UK year round with 10k men working at the sites year round, and 100's of millions spent every year doing so..... they win.
Terrorist attacks have always happened and will always happen, you can't stop it, not least because the more we react in bigger ways by going to war, the more hatred we generate in the world and the more terrorists there are.
Over reacting and making everyone here less free, while waging war and creating new terrorists is all just unbelievably stupid.