It's that this is the third or fourth lawsuit on Supes. A very bad precedent is being set. I don't want to be seeing this crap with other characters in the decades to come by other heirs claiming a sob story.
Do you even understand what the estates have sued for?
The litigation between WB / DC Comics and the estates of Joe Shuster and Jerry Siegel is with regard to the termination of copyright filed by them about a decade ago. If the court rules that the termination was valid (and copyright termination is a perfectly legal action under to the Copyright Act), and that the termination extends to derivative works, such as film, animation, etc., then Warner Bros. and DC would lose all rights to Superman and have to write an enormous check to the families for past-due royalties dating back to 1999 (when the termination was filed). It's not "another lawsuit," it's the same thing that's been going on for ages.
The other half of the coin is a lawsuit filed by Warner Bros. against their attorney, while claiming that he has a personal interest in his clients' victory. The attorney has counter-sued under California's anti-SLAPP statute. If he proves that the Warner lawsuit arises out of activities that fall under anti-SLAPP protection (essentially, that it's a malicious lawsuit), which is a pretty low burden of proof, then Warner Bros. will have to show that there is a reasonable probability that it will win on the merits of the overall case -- essentially meaning that the studio would have to put its entire Superman case on display, and then allow the opposition to respond. The judge could then throw WB's entire case out right then and there.
It's a bit more complicated than RARGH GREEDY HEIRS RARGH.