Whether it's technically legal or not is immaterial, it's still unsavory opportunism in my book. They have added no value, they're not looking to add value.
Which is basically property ownership/landlordship in a nutshell. Not that I'm disagreeing with you, just pointing out it's something we accept all the time already.
Like everything else, there's a spectrum, and I find the couple described in the article to be on the extreme "slumlord" end, except even more obnoxious because they are masquerading as nice gentle good samaritans.
In fact, they already did, by paying the debts of the property.
All of this was triggered by some super-mega-rich people not paying taxes for ~30 years, which after all this time amounted to a grand total of $900. After ample warning, the street was put up for auction and was bought by someone.
This whole situation could've been headed off at the pass several times: by paying the owed taxes at any point or by outbidding everyone at the auction as a last resort. But they took the "do nothing" route - maybe because they thought they could get away with it? - and got punished for it.
No sympathy from me. We all have obligations, and I get no shred of sympathy from my government when I fail to meet them (they slapped a €20 administrative fee on top of an unpaid €0,20 fine when they mailed the notification to me, just so you can get an idea... that 100x the cost of the fine in "administrative fees").
Why should they get anything more or less than what the average citizen gets?
Still, I don't have much sympathy. You'd think someone at some point in the 30 years they weren't paying taxes would have said, hey, you know, we haven't paid property taxes for the street in quite a while, maybe we should see what's going on with that.
If I had some problem stemming from failing to pay taxes for three decades, I doubt I'd be treated nicely either.