I am an atheist, but if I want to change my lifestyle or personality then I can practice any religion that I want. I know that there isn't a god out there, but I can take value in the way that certain people under certain religions live their lives. You don't have to believe in god to be a good person.
An atheist can practice some parts of Buddhism if he or she wants to. The only requirement for an atheist is that they don't believe in a god or gods, and probably that they don't believe in spirits or any wacky metaphysical beliefs. So they can practice the Buddhist ideas of mediation and moderation, but probably not the parts about ancestor spirits and gods.
It would depend on what branch and manner of practice. As long as the practitioner doesn't invoke a deity, then I don't see an inherent contradiction. Now a lot of Buddhists do believe in deity like beings, so it is possible for a contradiction to be present, but it can be avoided depending on practice.
Buddhism, while at first glance may seem like nothing more than a system of beliefs about a higher power is quite a bit more I believe. Buddhism is about a way of life and concerns itself with living a fulfilling life that is free from harmful practices that affect others. To this end, the belief in a "God" does not matter and an atheist can certainly practice aspects of Buddhism.