exactly - you have no idea why the friend laughed, could be loads of possibilities, as many of us have explained; she might not realise what the effect was, she might think it would be hurtful to bring it up again, who knows. None of us are mind readers.
People are not suggesting that the OP should try to understand the reasons for the friend's behaviour as a way of saying she shouldn't have been hurt in the first place. They are accepting that she was hurt, and that it is upsetting. BUT most people want to move to a place where they are not feeling hurt and upset any longer, and one of the best ways of doing that is to see the situation from the other side - to realise that there may have been reasons for it, that it probably wasn't done deliberatley, that the friend doesn't think she's a twat, that the friend probably is a generally nice person, that the friend might feel badly if she realised how upset the OP is, etc. By looking at all that, the OP can move past being hurt, which is surely what most people want.
Insistence on saying that you have a right to feel hurt, that you don't care why/how anything was said or what was meant, you still feel hurt etc, is just keeping you stuck there, and probably losing a friendship as well. If that's what you want, OK, but really, mostly people do want to move away from that, and posters are suggesting a good way to do so. It's totally different to saying 'you should not have been upset by it' or that 'nobody else would have been upset' or 'it doesn't matter'. It does matter that someone was hurt, and you can acknowledge it as well as suggest ways of moving on, of realising that you have misread the situation.
If you didn't think your friend was horrible and cruel for years, why would she have suddenly changed now? More likely is that there was an inadvertent reason for it, and she may not even realise how upset you are. This is not to say it was a good thing to do, or that it's wrong to be upset. But you can accept that it did hurt, at the same time as accepting that it might have been unintentional and that the friend wasn't trying to hurt - which is then the only way to stop hurting.