I can't fathom what reason there might be for a Terrorist group or otherwise to commit attacks on an event like the Olympics besides gaining infamy. If you have a specific target in mind that you want to enact vengeance or whatever on, wouldn't it make more sense to attack that group elsewhere instead of in an area where dozens of nations will be? You're pretty much pissing more people off by attacking an event like that, instead of focusing on whatever target you have in mind.
What is the reasoning for this?
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So they can affect as many people as possible.