The ban on liquids makes a lot of sense. You know it's water or diet coke, but it could just as easily be an inflammable liquid.
And while you may not have a canning machine at home, it would be very easy for a terrorist organization to have one and fill innocent-looking Coke cans with something else.
Most airlines offer free water/sodas soon after you take off, so you really don't need to take your own.