People are probably concerned because Facebook has an utterly atrocious track record for supporting developers.
(extreme hyperbole warning)
While nobody expects Facebook is going to just fire all the cool people and begin running things their way tomorrow, nobody expects the two entities to remain entirely isolated from each other either.
The real question behind it all: what can Oculus do now that they couldn't do without Facebook? Not Facebook's money
, but Facebook as an organization. Surely they've had suitors making offers left and right, so what are the hard reasons they, as people who just want to make awesome VR goggles, would choose Facebook?
The answer can't be as cynical as "their bid was the highest," but at this point it doesn't seem like anyone can think of any others. No one knows why this news should be exciting, so it must be bad. Perfect recipe for nerd rage (it was a Cool Thing that people Really Wanted that they Almost Had and now it's been Taken Away!)