Is it common for content creators to use donor-rigs? If so what are the legal implications? Are not rigs copy-protected?

I think it's common to get permission. You may not need it if you aren't selling the item, but if you are (or for freebies) you should.

Depending on the creator most donor rigs do come with usage information with in their read me files if that information is not available contacting the creator will get you the information you are requesting recently Smith Micro made announcement that **Poser Included Content Now Comes with Vendor Resources Info ** you can read their blog on this here I can not speak on Daz items as recently they have made many changes to how their content can be use. I hope this helps in answering your question.

using anything from someone else is not legal (unless stated it is)

Check their ULA ( licence) it normally says the way you can use the rig, re-selling, Etc

Sell and buy Amazing 3d animation for daz and poser

The policy over at Daz3D for years has been that it is okay to use a donor rig from figure to clothing FOR that figure ALONE. It is not okay to use it for another figure or a hybrid creation [for redistribution either free or $]. This is not legal advice. This is from somebody who has spent too much time reading forum threads ;-)