For example, in this revision to a question in U&L, I edited a question to remove a hotlink to a complete copy of a book which appeared not to be as far as I could tell in the Public Domain. I know I'm sort of asking for forgiveness rather than permission, but is this generally an accepted practice?
It's generally advisable to search for questions such as this on the main meta.stackexchange.com site given this issue is more broad than simply U&L. Looking for "copyright" and "links" reveals these Q&A's.
Also the FAQ can be helpful: FAQ for Stack Exchange sites
- Are links to copyright material allowed or not? [closed]
- Do I have to worry about copyright issues for code posted on Stack Overflow?
- Can we have documentation on copyright and fair-use with regards to improving link-only answers?
This 4th link is interesting, the answer is shog whose actually an employee of SE.
For what it's worth, the relevant help page doesn't say anything about copyright because there's no "safe" way to copy - if someone wants to be an ass about it, they can go after you for copying a paragraph as easily as a page. Doesn't mean they'll win, but you can be pretty sure that everyone loses time and money waiting for a fair use ruling.
And that's ignoring the differences in copyright laws world-wide. And DMCA. Y'all aren't lawyers and neither am I - so frankly, the less said about copyright the better.
What we should focus on is plagiarism: this is not a legal issue, but a matter of etiquette, and one that I think is pretty easy to apply: we ask for attribution from folks copying stuff posted here, so we should extend the same courtesy to stuff copied to here. Some communities are more strict about this than others when it comes to the amount of text being copied, but at minimum you should always require that the original author is credited for his work. It's just good manners.
Now, for the link-only delete reason... It's pretty close already; maybe:
This link may answer the question, but answers that depend entirely on an external site can become invalid if the linked page changes. It is better to include the essential parts of the answer here (with credit given to the original author) and provide the link for reference.
So given the above I'd recommend calling it out if it's a link-only-answer as such and by addressing it as a link-only-answer, some portion of the link would need to be moved into U&L to make it more complete and tolerant to the link going away in the future.
By moving it in, if the material is not meant to be here due to copyright restrictions, it should be deleted/closed.