Maybe, if OP's DS has been regularly bullying their DS, they wanted to make sure they could be there to watch and protect their own DS at the party - and then, OP's DS was continuing to bully their DS at the party and they saw red.
Of course, none of us know this - and it may be that OP's DS is absolutely not a bully and has been falsely accused; indeed, as has been suggested, their DS might have been bullying him and it's classic DARVO in action.
The adults probably shouldn't have approached him this way - although I don't see how soundly chastising a bully (if indeed he is) and warning him to stop actually counts as bullying itself.
I know a lot of people will scoff and ask how a 7yo could possibly be a bully, but some kids have indeed learned how to be really horrid, nasty and manipulative by that age - sometimes deliberately exploiting their young age and bursting into fake tears in order to get away with it.
Again, none of us know the full facts here; but some children of that age ARE nasty bullies and many schools are simply not interested in acknowledging that they have a bullying problem, much less in resolving it. It's far easier to ignore it and/or to tell the victim that they 'need to learn resilience'.
Those parents could have a perpetually very upset child whose life is being made an absolute misery by a nasty bully, have reported it to the school again and again and been repeatedly brushed off and nothing ever done. It happens a great deal.
If that were the case here, what choices would the parents actually have apart from to force their own innocent DS to move schools, put up with the bullying (and ban him from attending any parties where the bully might be) - or to take the opportunity to advocate for their own child and give the bully a very strong warning to stop?
More facts are needed here; but many bullies are skilled manipulators - even at that relatively young age - and are hardly going to admit to adults that they are bullies and be stopped or sanctioned.