As a mother of a four-year-old little boy who hears everything and repeats it, I have to really censor my language around him. I never use the Lord's name in vain around the general populace out of respect. Same way I try not to use the "bad" swear words in front of others because you never know who may be offended or if there's children in close proximity. I don't say the bad word using God's name in front of anyone except my husband and it's usually done when I'm in the throes of an angry rant about a particular subject.
Why would someone who doesn't believe in God curse his name? Not sure. I really can't explain the psychology behind it. I know for me it's just language I've heard all my life and I guess it's just sort of a habit. I wouldn't say it's ingrained, however, as I can exercise restraint over my choice of words any time I wish. Every so often I'll slip and then offer my apologies to those around me, particularly in mixed company. It's simply being tactful. I'm not one of those people who say, "well, if someone's offended too bad!" I can at least exercise couth and discretion in public and around my family.
Uphill Battle, I can understand how you as a believer would be offended by the taking of your Lord's name in vain. I would hope that those closest to you who may or may not be of the same faith would be considerate and not let the blasphemous swear words fly; however, people of all stripes use these terms (even some Christians, particularly when they get angry) so we really can't try and dissect the logic and reasoning behind any group of people choosing to use such choice words and phrases. It's just the way life goes.
And to answer, in the simplest of responses, to why someone who doesn't believe in God would choose to use his name as part of a swear would be similar to asking why a diehard feminist would ever use the "b" word. It may seem like a contradiction, but really it's just the way things go sometimes.