hm...the issue with the license here is that a lot of free resources don't have a license description. I noticed this from the start. This can be very misleading to people who intend to share foreign resources. As I can imagine most people don't bother with this legal matter, they may think that resources that don't have license descriptions can freely be shared, but this is in fact wrong. The truth is, if somebody shares a resource and don't provide a license description, then by law the "All rights reserved" copyright applies. Which means that as an owner he has exclusive rights to his work. Due to the xenforo.com user agreement this right will be shared with the xenforo owners, though.
Since resources aren't only add-ons, but
tutorials also, mirroring them without getting permission in the first place is against the copyright law (US, UK etc. are in this regard the same).
All this can possibly be resolved if Xenforo extends the resource manager to allow authors to specify the license of their resource, not in the description, but in the sidebar.