Say you're talking to someone who can teleport instantly across the globe. If you tell him where he can see the Mona Lisa, that's OK. If you grab the Mona Lisa and wave it in his face, that is very un-OK.
Before someone replies with the "but digital items can be copied, it's not stealing", that's the argument most piracy enabling forums and websites are able to get away with. If you host, EG, Nintendo ROMs on your website, you'll get shut down quickly. If you just save links (URL or Magnet), you might get a DMCA for a specific item maybe, but won't get shut down entirely, probably.
As always, there are a lots of exceptions and alternative interpretations though.
Why not just charge the 100th visitor per minute or something? Someone will be motivated to pay the cost. And, if people don't want to pay? Well, even those with less motivation will find a way to the content somehow.