We really don't. I hope you don't sincerely believe your fellow HNers are stupid enough to think that copying an artistic artifact somehow removes it from the possession of the original creator. We aren't and we don't. Which means this is just a matter of word choice. I can't think of much I'm less interested in reading than "You used this word in a way I don't agree with."
Besides, this usage of "theft" is not dissimilar from the already well established usage of "stole" as in "he stole my idea." If an idea can be stolen, it can certainly be theft to take the product of an idea, at least colloquially.
Of course it's a common usage, but that alone shouldn't make it acceptable usage. Lots of disparaging language is common usage. When you use a word with a certain intent, it's the usage of that word with that specific intent that should be questioned.
I think we should all be grownup enough to recognise that ideas can't really be stolen either. We also recognise the value of copying ideas. If we recognise these things, then we should use corresponding language to reflect our intent.
Besides, this usage of "theft" is not dissimilar from the already well established usage of "stole" as in "he stole my idea." If an idea can be stolen, it can certainly be theft to take the product of an idea, at least colloquially.