Susie - there's no justification for it, its assualt. I always tell women before doing a VE that if they want me to stop to tell me and I'll stop. If they ask I stop, even if I haven't managed to get the dilatation.
As a student I saw a midwife carry on when a woman was begging her to stop and found it rather awful. In this case the midwfe justified it in that the baby was in distress and she felt she really needed to ascertain what was happening as a matter of life or death.