I'm at work so hadn't heard the details yet but if they did ban him for life I'm sure they have more on him then just this private conversation. Let us not forget his issues with the ex GM Elgin Baylor, he has had previous comments and run ins that for whatever reason never got as much attention as this, as well as I have also heard he had run ins back when he was part of low income "slum housing".
So if they have proof of all that they can use that history has a repetitive behavior pattern and hopefully make it stick. Also whoever said it, that this is bad business for the NBA and they can use that his actions are causing them to lose sponsors which = money. Cause as bad as this recording may have sounded, it is a private conversation and it isn't exactly against the law to be an old bigoted ahole.