Why do some people say this name is spelled wrong?

Rebekah is the version used in the Bible. It is the Greek spelling. However, most people in the US use the Latin spelling, Rebecca. So people say it is spelled wrong because they are more familiar with the Latin spelling. Personally I prefer the more common Rebecca, but you are right that Rebekah is also a perfectly fine spelling (if you want the name "spelled right" meaning the original spelling, you'd probably have to use a different alphabet, for one thing, and for another it would probably be Rivkah, or something like that).