Whether CDL is legal under Fair Use or not was/is legally unresolved. Whether it's legally permissible to copy a printed material and distribute that copy as though it were the printed material was/is unresolved.
Lots of organizations have been skating along under one interpretation of Fair Use as a workaround for the burdensome licensing fees and absurd lending limits that publishers expect libraries to pay. Libraries being, y'know, super well-funded by that VC honey and all.
And it's not like publishers get less litigious over time. Whether the NEL prompted this or not is irrelevant; it was going to be tested, and IA are exactly the right organization to test it, and it wasn't going to become "more legal" if everyone just kept doing the same thing but more quietly.
Now that it's being tested, the right thing to do is to figure out if CDL is something we as a society want to enable for our libraries or not.