The official statement said that $2,000 was in line with what other fines were for similar offenses, and that she didn't cross the line into a major infraction.
Part of the problem is that it's a repeat offense. Every other sports league in the world steps up the penalties for repeat offenders.
I was listening to someone (podcast, radio show, forget what exactly) and they made an excellent point. Tennis players have more power during their matches than just about any sport, and they know it. There's no bench. You can't just kick someone out of a match and substitute them with someone else, like you can in basketball, football, etc. It's not like track or swimming (not that this would ever happen in those sports, it just doesn't make sense) where there's dozens of competitors. So Serena can say just about whatever she wants, because they're never going to kick her out of a match.
Women's tennis has so few stars right now, they're terrified to suspend her or force an apology. They're screwed, and she knows it.