Why wouldn't it be? Because is a subordinating conjunction, used to introduce a subordinating clause. Because a subordinating clause can begin a sentence (such as this one), it is certainly correct to begin a sentence with Because – if the thought is then completed with an independent clause. "Because it rained for two days, every street in town was flooded." The prohibition against beginning with because probably derives from teachers' attempts to prevent students from writing dependent clause fragments in which the thought is left incomplete. However, no such restriction has ever been placed on beginning sentences with Although, If, When, While, Since, and other words that, like because, can also function as subordinating conjunctions. Why because has been singled out for this dubious distinction is a mystery.