The building in the linked article is in a Nationally Recognized Historic District. So in that case, it's plainly illegal.
In the not-so-illegal cases, it's another force of entropy dragging the city down for a quick buck. A beautiful old fixer-upper of a house built by some long-dead furniture baron is cool in its own right, but if all the mahogany wainscotting (and everything else of any value) is pulled out and sold across town or across the country, it's that much harder to sell the building; to fix it up; or even to argue against demolishing it.
Detroit is full of structures that in most other places would be refurbished rather than demolishing them. But here, the momentum is solidly behind tearing shit down and building something new. Compounding the problem, rent even in the most in-demand neighborhoods is too low to support high quality renovations.
It's a complicated issue. I'm generally in favor of preserving building stock -- modern buildings rarely are as beautiful, or as solidly built. Old ones have a lot of idiosyncrasies and outright problems, but they're often literally irreplaceable -- they were made by skills that we do not practice anymore. At the same time, there is a point where the number of vacant structures is overwhelming; they are often very dangerous,
even just to passers by; and you can't compel other people to spend their money on projects they don't support.
In fact, that's actually the underlying challenge with this town: it's so damn complicated and inter-dependent. It's impossible to find a single overarching solution, or even a clear solution to a discrete problem. A lot of horrible solutions -- short-sighted money grabs or just plain foolishness -- are proposed (and often attempted.) Which leads to a ton of inertia against doing anything unless it's known to succeed, since there's no need for more fuckedupedness. Which is basically just the road to stagnation, which at this point is really just active, accelerated decay.