I don’t know where that leaves you as a business. I suspect this leaves Apress at the "Require all further authors to contractually promise us, in writing, that they are assigning Apress the exclusive right to publish/distribute the content and have not written any legal landmines which contradict this clause into the sections we routinely don't read".
I've noticed a number of technical books developed in an open manner (though usually not under the GPL) that then sold well. There are also a lot of good people around software these days who strongly believe in the ideals of free and/or open source software. Therefore I suspect that insisting on strict copyright control will lose more sales than it gains. But people will make mistakes as they learn this.