It drives me nuts to read everywhere - from Kühnen, von Wadenfels, the "journalists" - that the guys should realize that the exposure they would get in DC is more important than Indian Wells - as if they didn't know it!
That's precisely the point, it couldn't be more obvious that they have nothing to gain and much to lose doing that and that they must have serious reasons to do so.
But no, everybody seems to content themselves with the paltry excuses and Kühnen's "es ist ja nichts Außergewöhnliches, dass Spieler absagen". ^^
I just came across this article of Philippsen who says that Kühnen actually wanted to take Haas, Kohli, Petzsche and Zverev to Toulon - not sure why he wanted to have Zverev rather than Berrer, although it might be a good thing indeed to have him on the team since he's younger than the others - he would have been a better choice than Greul anyway:
She does stress that the given explanations are "teilweise fadenscheinig", that Kohli's "outspoken" style is not very popular among his peers and neither is Kühnen's way to run the team, but that's pretty much it.