Or then the device had no planet-creating ability (it never was credited with it, after all), and merely put life on a lifeless heavenly body just as the sales pitch said. Thus, it would have turned the surface of Regula to the lush and fertile Genesis Planet, but when hitting the Enterprise
it would just have turned the ship and its crew to a sperm whale and some petunias, all of which would have quickly died as they didn't have a planet to stand on.
There might also be some focusing issues, so that the Genesis effect would only create viable life at the point it was targeted on, but mere vaguely lifelike not-quite-shapes in those areas of space it passed through on its way to the focal point.
It should be noted that Spock was not rejuvenated by the wavefront of the Genesis effect itself, but by the aftereffects several hours or days after the Genesis Planet had been established. Presumably, then, the wavefront would not have rejuvenated people aboard the Enterprise
... Unless "aftereffects" and "off-focus effects" are more or less the same thing.