That’s a weird reason to give for it, like it’s obviously not going to change anything. The Justice system isn’t going to be held up by an artist threatening to destroy some paintings (and it could be years before Assange ever dies in prison), it seems like it’s being done as performance art. Pretty much like Banksy trying to destroy one of his works right after it was sold at auction, it’s being done for the attention/message of the act itself.

If there is more of a taboo around destroying art over human lives, it’s not for the sake of the “Art” itself, it’s for the sake of the arbitrary, yet quantitative value of money that those works have attached to them, because of how ridiculously inflated the price of artwork is (for money laundering purposes or whatever financial hijinks the wealthy are using artwork for). The historical value to humanity probably plays into it as well, but that’s not nearly as big a consideration. Otherwise though, you’ve got governments like the Taliban which will happily destroy history just as easily as they’ll kill people, that’s the kind of society we could have where art is given equal weight to human life.

Meanwhile, the people that our society deems worthy of death are people who don’t really have much income or wealth to their name. If you looked at the average wealth of individuals on death row or who have been executed in the US, it’s likely to be lower than the wealth of average Americans. So Art is valued more than human lives because, quantitatively, some pieces of art are probably worth more than some people would make in a lifetime.