Either an
excuse used by someone who wants people to
ignore whatever it was they just did, or an
assertion by someone who
realizes that they cannot become something
beyond what is
presented to them by the
society they live in.
Society doesn't completely make a person what they are. Genetics have quite a bit of influence also, which is being shown more and more (such as experiments with identical twins).
But it does have a huge effect. Your personality and attitudes come out of the society you're raised in and the messages you're exposed to. One could call it brainwashing in a way, except that in that case we'd all be brainwashed, as none of us can avoid the influences.
Your family, your friends, your school, your neighborhood, radio, television, the internet - all of these have their own effect on shaping who you are. And if you're never exposed to certain things, you'll never be influenced by them.
A child raised in the inner city who's told by everyone around that they can't get out of there, that education isn't important, and never has anyone really tell them that they can, will probably never even consider getting out. It won't be an option to them.
Some people don't believe in free will, and think a person is entirely the sum of genetics and social influences, and due to the complexity and the inability to do any tests with human subjects from birth, we really can't be sure if that person who just committed murder really chose to, or if the sum total of their life experiences are the true cause - after all, just about every serial killer had a horrible childhood and you can almost see how they couldn't avoid being that way.