I can fully understand why he doesn't want to come back to CSI. He made a concious choice to leave that show for a reason - no matter what the reason is - and I think it'd be awkward for anyone to have some sort of expectation that he "should" come back, even if it's just for one episode or so. After all, if he'd been a "normal" person who quit one job in an office and got another job as a teacher no one would even the slightest ever expect him to drop his teaching job repeatedly to go back to filing papers in the office. And although acting isn't the same, it is still a job. I don't particularly find William to be disrespectful or usavory towards the fans. He's no longer a part of CSI, he has no obligation towards anyone in regards to it. If he wanted to guest star then that'd be cool, but he doesn't want to and that should be fine with everyone.
And when it comes to the GSR angle and how Sara and Grissom aren't so linear anymore now that William has stated he won't return again, well, that's all on the writers and NOT on William. They made a concious choice too, to continue a relationship between a star still on the show with someone who wasn't. What they do with Grissom now is on them and not William.