I'm uncomfortable with the idea that people who don't post images should be treated differently.

Just looking at it on a practical basis, I can't see why using the Favourites feature would be an indicator of potential theft. If I wanted to steal something, why wouldn't I just save it at the time? Equally, I can't see how a mail to a member who has a lot of faves would elicit anything other than "Well, I like your image." I can't honestly see anyone replying "I plan to rip off your sig, add some repulsive animated sparkles, add my own name, and post it all over the net! Bwah-ha-ha-ha!" You know what I'm saying?

How about if members had the function to say whether they allowed favouriting on a case-by-case basis?
Like when you upload:
Allow viewers to make comments on this image? (Crit, Non-Crit, Both)
Allow viewers to rate this image? Y/N
Allow viewers to add this image to their favourites? Y/N

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"you are terrifying
and strange and beautiful
something not everyone knows how to love." - Warsan Shire