Yes, that is essentially the reason. It's not hard to know enough chemistry to figure out how to make these things. The fact that such attacks (your example is small-scale and very ineffective, let's not forget) don't happen more often is the general incompetence of human beings and the relatively tight controls on the basic components (which aren't particularly challenging to monitor for). The tests described are theater, based on the idea that knowledge itself is dangerous.
This way of testing is a regressive stance that essentially presupposes that our adversaries are dumb babies that can't figure anything out on their own. If that was the case, they would also be too stupid to figure out the correct things to ask to get a real set of instructions. Given those things, it's theater.
Theater wastes everyone's time so that people who cannot or don't want to evaluate the actual risks involved. This is something we shouldn't make a habit of doing. It's not worth wasting the time of people with good ability to assuage the worries of people with little ability in a way that has no effect on actual risk. Instead of this, we should address real risk (which we're already doing) and educate other people so they can understand that these are the correct steps to take.