I'm A Banana
Potassium-Rich
I see this a lot. From a legal standpoint that's true (since you aren't depriving anyone of what you are taking). However, from a conversational standpoint "you know what we are talking about." It's a nitpick that doesn't add to conversation, it just sidetracks it into an irrelevant direction (unless you are discussing how it works in the legal system).
Even from a conversational standpoint, theft implies that someone else has something, then I take it from them, leaving them with nothing. In the case of copying something from someone's spellbook, I'm not TAKING anything. They still have the original spell, and now I do, too. The only people that miss out are the middle-man who collects a fee for spell transfer, and the original spell creator, who misses out on credit and payment for the spell's invention (assuming spell copyright works something like real copyright, anyway).
So calling it "theft" isn't legally correct in the real world or even conversationally accurate to what "theft" means. I mean, the spellthief doesn't just copy a critter's spells...he TAKES them so that the creature CANNOT use them anymore, robbing it of power as he powers himself up. It's the difference between healing and enervation, between sharing and stealing...there's a pretty big conceptual gulf between copying something and giving it away and taking something from someone else.