No brainer? If everyone is starting to arrive from today then the site needs to be protected from today. Simples. Some Israeli team members flew in to Heathrow today for the Olympic village - do you think it would be acceptable to have athletes unprotected for up to 2 weeks before Games?
Furthermore - putting the airspace restrictions in place also helps iron out problems such as identifying pilots who haven't bothered reading all the literature and perhaps sorting out the fact that Atlas Control seem to only have one phone number (they we're engaged for most of today!)
And Mr Al Q isn't the only threat. These groups know that the games are a hot topic and security will be tight. Disrupting the games before they actually begin might also be seen as a 'prize' so lets not make it too easy for them eh?