Either they were going to be shut down by their own choice, or their hand would be forced by outside litigation. Just because something is legal doesn't mean companies won't spend millions of dollars to make your life miserable because of what you built.
Another way of looking at it is that, as things stand now, the legal system is just another weapon the rich and influential can use to crush the little man.
Just because your product is legal, but mainly used for illegal purposes really opens you up to a lot of issues. They should have expected this was coming.
If not, then you were illegally distributing the film while you were watching it using PopcornTime.
The fact that this is still legal is largely historical accident, and is bound to be overturned by one lawsuit or another, or Congressional loophole closing. It's still morally a crime.
Your claim is no more valid than saying if you tell your friend to park their car illegally, you are an accessory to fraud.