The developer is involved to the extent that they have to do what Sony says. The CEO of Arrowhead even tweeted that people should email Sony, referring to a Playstations own website where it stated that PSN is optional. Sony then quickly changed their web page to say it's sometimes required.
A change like this may warrant special exceptions to the refund rules. Steam has made exceptions for similar changes to games made after launch in the past.
That's true. Personally though, it not being optional for any amount of time just shows that there's no good argument to have it be mandatory in the first place.