The SEN boards (and I know a few personally) are full of people sending their kids to independent schools because of the small class sizes, better pastoral care etc.
The 'private sector' (just like the state) are all not the same. The likes of Wycombe Abbey, Eton etc are not places people go for the 'best' education. They are elite boarding schools to rub shoulders with the upper crust. The 'best results' are just a cover - I'm sure loads of 'average' people if spoonfed could great great academic results too.
If I was this man I'd be very angry that I entrusted my daughter to a 44K a year school and this happened. But then again, there was nothing stopping him from sending her somewhere else with a more welcoming environment, was there? @IBlinkThereforeIAm you are right ordinary parents have little choice, but not this family.
I'm going to add as well we don't have the full story. We don't know what the school did and didn't do. She wanted a 'suspension;, not a detention. How did that go? Did they just not want to waive the punishment? What alternatives did they have? Regardless of their best efforts this could still have happened. So it's very hard to 'point fingers'.