I believe the thinking was bamboo was just too far from anything reasonably historic. I know laminated bows are nothing close to being historic either, but I think the warbow societies are looking to avoid people showing up to events with horse bows, or reflexed or recurved bows, or anything that doesn't closely "look the part" of a warbow. There are also minimum draw weights and OAL for bows, and no horn or sinew is allowed.
I know a maple or hickory backed yew warbow is difficult to distinguish from a yew selfbow, until you have a closer look. I guess you have to draw the line somewhere, and they've done it with bamboo.