Presumably, because they believe good schools should be freely available to all, but in the mean time are living within the system they find themselves in?
I pay for private dental treatment for dd - the nearest NHS dentist who will take her on is 30 miles away, and it would cost me more to take time off work to get her there than the Denplan fees. Does that make me a hypocrite - I believe there should be good dentists available to all on the NHS.
Similarly, I spend a lot of my life campaigning for affordable, local, sustainably produced food to be available for everyone. In the meantime, I'm grateful that I can afford to buy good quality food for my family. I don't feel the need to live on cheap processed alternatives, even though many people have no option but to do this.
I'm actually really sympathetic to Diane Abbot specifically in this instance. Black children, and particularly boys, suffer massively from institutional racism in this country. Why the hell shouldn't she choose to level the playing field just a tiny bit for her son, the same way she probably pays for dental treatment for him or makes sure he has decent food. She's worked hard throughout her life to try to get good opportunities available to all black children in London, it's not her fault that so far she hasn't succeeded.