Most of the time, there isn't any danger in pushing on a cervix that is not, according to the last VE, "the magic 10". Because you could easily have dilated to a 10 since then and even if not, it may help you dilate the rest of the way.
Sometimes, pushing against an undilated cervix causes it to swell. I think it might be that people thought it was dangerous to push early, because their understanding of pushing was the PURPLE PUSHING - I can see how doing THAT on an undilated cervix could cause problems! But intuitive, gradual, grunty and intermittent pushing at the peak of a sensation that most ladies tend to do as they move into the pushy phase of labour.... Well, it's not comparable to the "chin to chest, bear down hard to the count of ten!!!" style of pushing still done in a lot of hospitals...
Sometimes, women UNdilate during a VE so, it is always possible that you could be at that magic number 10 they like so much, and feeling a strong pushing urge.... and they check yo uand the act of doing that causes your cervix to undo that hard work you've put in and they tell you the crushing news that you "can't push yet, you are <insert umber lower than 10 here> cm".
SOMETIMES your cervix dilates past 10! I know, I know, shocking isn't it? Everyone's bodies are different, all different shapes and sizes! And SOMETIMES (gasp!) the person doing it just gets it wrong.