In a way it's remarkable that this is the first time that we've had something like this happen - since the threat of terrorism really came on the agenda over the last 15 years or so, something like this has always been a danger. No chances can be taken, and all precautions are necessary.
And once something like this happened, it was always going to be targeted at United, since - despite currently being shite - we are the biggest and most popular name in the game, with the largest stadium. There was talk of a plot targeting us in like 2001 or 2002 IIRC for that reason, though I'm not sure if that was ever based on any real threat - anyone remember?