On the surface sure it looks like an oxymoron, but there is an explanation:
They're probably in the minority, but some Wiccans (and plenty other denominations) believe in all of the saviors throughout history and across all religions. Apollo, Zoroaster, Buddha, Osiris, Jesus, and many more are believed to be equally valid and sacred; variations on the same story where the name changes with each time and place. Exactly like there is a transcendent, transtemporal, transcultural, teacher, leader, healer, miracle-working, savior archetype. (And who's to say this isn't one hero being reincarnated?) This belief holds that whatever you choose to call the savior doesn't matter because it's basically the same faith in a good higher power versus a bad, twisted power. The rest of the details are superfluous and to focus on them is missing the point, thereby twisting the meaning, thereby bringing about the evil, such as holy war. Wiccans believe in a higher power (divisible into various forms), and Christian Wiccans choose to call it God out of convenience. Perhaps they even look to the Bible for advice sometimes.
Many, if not most Christians have engaged in premarital sex or coveted their neighbor's wife, but nobody says they can't call themselves Christian. Why should one group's selective adherence to biblical instruction be acceptable while the other is ridiculed?
Also I think it's worth noting that Wiccans are aware of how the public views them. Faced with that, it takes a lot of conviction to call oneself a Wiccan. You can bet, they really believe, they really thought things out. Their spirituality is not to be taken so lightly. The spellcasting is hokey hocus-pocus, yeah, but I don't even think Wiccans believe their spells are gonna work, so much as provide a form of ritual spiritual practice for them.
Spirituality takes many forms and as long as they make sense to the believer, they all make sense. After all, religion is the search for answers to inherently unanswerable questions. The Wiccans' guess is as good as the Christians'...