The "in a manner" that you omitted is crucial here. The purpose of the site doesn't have to be for doing the illegal things. The purpose of the site just has to be something that happens "in a manner" that enables or facilitates the illegal things.

So YouTube - primary purpose is uploading videos, not illegal. But YouTube doing that in the "manner" of allowing anonymous internet users to upload anything they want - facilitates the illegal things.

So I disagree with you based on those few words. Even if you think your point stands, I'd posit the language is so dangerously ambiguous that it should be a problem in itself that ensures it never makes it into law.