I don't have any inside info on this, but my assumption is that the pages are being accessed by automated programs that are accessing many pages simultaneously (to what end I don't know - search engines updating their databases, possibly). Registration would limit page access to those with credentials, and robotic programs wouldn't have the credentials.
This could be accomplished with a free login, and my assumption is that the registration will be free.
Xwordinfo already does accept donations, and the suggested donation amounts are (if I remember correctly) $10, 20, and 50, so my assumption is that even if the login came for a fee, it would be extremely reasonable (e.g., $20 per year). But again, my expectation is that this *will* be free.
I agree, that website (and this one) are two essential references.