Because smoking kills people. In an enclosed space, the fumes get everywhere. If in the concourse and you are smoking then your fumes will reach me wherever I am. You are a typical ignorant, selfish smokerYou have the option to go elsewhere, no one is making you stand next to a smoker.
Why should a smoker not be allowed to smoke purely because others dont?
It's a silly law that is discrimating against many people. In other aspects of society, many are massively sensitive to discrimination; if the anti-smoking campaign goes any further I think its only a matter of time before people who do smoke react to it.
The most sensible option would be to have smoking areas within a place like a football ground, gives people the option. Without it, you are taking away that option.