Well, as far as i see, the biggest problem is if this is done against the artists explicitly given permissions. If that happens, it could be considered copyright infringement, which indeed is a problem. If there are no explicitly given permissions, well, it's
partly the artists own fault for not negotiating these to begin with, and also partly the customer being a dick for not asking before distributing.
Another factor to consider is whether the pay is for the work done or for the product itself. Commissioning an artist is the former, visiting an art store is the latter. If you've commissioned an artist to make something specifically for you, chances are that they'll be a lot more lenient about what you do with it, or at least more negotiable, since you can discuss those things before work actually begins. If you're buying from an art store, the artist might not even be around, and in that case, my opinion is that you only buy as many instances of the product as you pay for, and while you can give these to others if you want (and thus stop using them yourself), copying is probably a no-no unless otherwise stated.
Of course, the fact that digital copying is
so easy makes it very tempting to just copy-paste to your buds and not think further of it. That, however, does not automatically make it legal; video game piracy works the same way, and is definitely NOT legal. I mean, if we are to go by technicalities, nothing is set in stone unless the artist puts a license on their work, but it's pretty safe to assume that if you've bought something in a store that isn't made specifically for you, you're probably not allowed to copy it. If, however, the artist says "sure copy as much as you want", then by all means, do exactly that. If nothing has been stated, it's kind of a gray area, so you should probably ask first if you can.
Long story short, communication is key, and
please respect the wishes of the artist(s) even if they haven't put an explicit license on their stuff (one such way to do so is via
Creative Commons if you're curious, although that particular license does promote free sharing). I doubt that we'd ever have to contend with dead artists in this community, so don't worry too much about that part. :P