Though to clarify, I don't think we covered anything in the tips & guides section. Authors have the ability to delete their own resources if they so choose.
I noticed that too this morning, and I thought that that was unnecessary. Documents posted in Tips & Guides are not installable, and it does not matter if their authors are active or not. It accomplishes nothing in my opinion by deleting them.
I have edited the sticky thread to explain why free resources may be deleted.
There may also be occasions where we delete free resources, if those resources are associated with a member who is a known pirate, for example.
In general though, any free resources which are deleted will have been deleted by the author or at the author's request.