I don't necessarily think they are vindictive. I think they probably just come across too many families who operate under an "attack is the best form of defence" mentality that they are often just suspicious of the real reason behind it all.
I work in schools, not social services, but it happens often. A new child starts at school, very quickly there are a few little red flags which get noted. The family is on your radar. Before social services are even called in, you can guarantee that the parent has already complained themselves on more than one occasion about the school, it's any excuse to have a go. About the way they've been been spoken to by this member of staff, or that member of staff, didn't like their tone of voice, felt they or their child was being treated wrongly or unfairly in some way, or they themselves were being harrassed about attendance etc. The class is being constantly covered by supply teachers, they haven't been given enough homework/they've been given too much home work, anything and everything. It's a sad situation, they are just feeling desperate and lashing out as they know things are about to come to a head for them.
Anyway, that's all nothing to do with the OP's post.
I do find the treatment of OP over the top and wondering if it's not just the bruise on its own which has caused such an OTT response. I mean, fair enough - follow protocol and investigate, but if you don't have the resources to do it in a timely fashion for all cases, both one-off benign accidental ones and ones where there is a real concern over safety, then you should only be focussing on the ones where there is real concern.
And social services clearly DON'T have the resources. Time and again our safeguarding lead is batted away/regular social worker is unavailable and doens't get back to her for days when she tries to report an issue. On some occasions school staff have been the only authority to turn up to a child's home well after school hours where there is a serious concern over the safety and welfare of the child or the parent. It's clear that social services are so overwhelmed that in some cases as long as they know a responsible adult is with the child at that time then there is no need for them to give it any immediate priority.
In the cases that make the news, the real issue seems to be the fact that concerns are raised multiple times, and no-one follows it up, or shares information appropriately. Or there was no consent from families so the silly systems says social services couldn't take it further etc. You can bet in those cases there were multiple red flags, there is a whole history. In OP's case it sounds like this is a one-off thing so I have no idea why they have jumped so hard on her.
As a loving, responsible parent, it would be going against all my nurturing instincts to have my tired child hanging around a hospital ward for hours and hours, out of their routine, waiting for someone to be available to look at ONE small bruise, with no accompanying concerns, when they could be safe at home getting proper rest and see someone in the morning instead.
Fair enough, do the checks. But for god's sake, put the resources in place to allow them to be done in a timely and reasonable manner. It is totally wrong to keep parents and children waiting so long when they have consented to be there to tryto abide by regulations.