According to people close to the Olympic Opening Ceremony team, who has worked with Zhang and Spielberg, there is another reason for the resignation, i.e. besides that infamous pressure from that poorly acted Daisy who was responsible for taking our great Gatsby from this world.
While the PRC claim that Spielberg's contract has never been sign was true, as we all know, it was just an excuse and make his resignation easier, i.e. without contractual/legal consequence.
The Olympic Organising Committee was actaully in chaos, as bureacrats were running it (instead of businessmen, or professional event organizers). Progress was slow, and while budget was not stingy at all, the funding process has been sluggish (as a result of bureacracy).
While many artists are willing to sign up even if there is no pay (just for getting the resume credit), they were not being paid for about 6 months. But Mr S does not need this credit and there is no reason for him to do volunteering work. This is why the contract has never been signed, and why he finally got Daisy as an excuse to get out.