Sharon,

With respect you are speaking rubbish:

"they probably didn't realise there was a (presumably) strong risk of a terrorist attack when they bought their tickets."

The terrorists were not focussing their efforts on Stratford 'irrelevant' of the Olympics and suddenly realised 'Wow isn't this convenient!'. Anybody wishing to cause mass destruction does so where, surprise surprise, they will cause mass destruction - E.g., a specifically built up area (possibly, by chance because this is a build up of people watching a particular once-in-a-lifetime event).

The Olympics could have been in a Paris, France. Guess what? Paris would have become a higher priority target for terrorists and anybody with the capacity to worry about the risk would have also have the capacity to consider the (obvious) risk when attending a high profile sporting event such as a the Olympics *before* purchasing tickets.