But it does make realistic sense, because of what Vivi and I pointed out. The Federation does not have the resources to stop a spaceship careening towards Earth, and given their history there is no reason they would think such a thing would be necessary. Certainly if you disregard the backstory of the Federation it seems ridiculous, but Earth is in the middle of Federation space and apart from Nero's attack in the last film, which they are likely to have thought was an isolated incident due to their complacency that is clearly established elsewhere in the film as well, hasn't been attacked for a hundred years. It also makes sense on a meta level since they were pretty obviously trying to evoke 9/11 (indeed they dedicated the film to 9/11 first responders iirc) and the U.S. was similarly complacent at that point.