It is a change, but I don't agree it is a problem.
Speaking only for myself, I am put off by sites that are indexed, but when you try to read the whole subject (i.e., look at attachments) you have to register and give personal information. My first impression from such sites is that they are in the business of selling that information. I generally go somewhere else and don't register. Assuming the site allows access to posts for nonmembers, I can't think of a credible reason not to allow access the attachments as well. (That applies to sites like ETO. There are sites that have proprietary or copyright-protected information where an index is available, but the full report is not. IEEE Proceedings is one example. But, that is not a forum like ETO.)
On the other hand, if the attachments had been available, I might have been impressed enough to join the site and try to contribute my share.
John