No, the argument is that the incentives aren't really there for other possible groups to commit this crime since they have legal alternatives to what was done. Doesn't make it impossible, just less likely.
I may be mistaken, but I think the argument being made is more like, "no one commits crimes, or any acts really, without a motive." These people, at least their leadership, have significant motive to not commit an act like this.
My money would be on terrorists as well, but that's why we have investigations. Let's see where the satellite photos take us.
But that's the same argument, right? The "motive not to commit an act like this" which the article provides is fear of prosecution; so if this argument went through it would just as well be an argument against any crime ever occurring.
Why would anyone ever rob a bank when you can earn money legally?