The fact is that US tax returns are private information, regardless of what other countries are doing or what you and I think should be done.
The same way the Snowden leaks were instrumental into regulation about surveillance, leaks like this are instrumental in crafting better tax policy, so I would say they are a public good. It's also not-quite possible to do it via completely legal means (as vested interest opposition is too big and has too much capital - the only way to battle against them seems to be to do it via not-so-legal means).
So in this case the fight is punching up against people in positions of power, not down onto people that have no power, so I don't see a direct issue with it. If you have power, you should also have the proportionate responsibility.
Snowden leaked government program details. How in the world is that the same about thousands of private citizens (aka not public figures) personal financial details?
How are these individuals not being "responsible"?
The only unresponsible person is the person(s) who leaked the details, and ProPublica for monetizing it.
If someone is powerful enough to have power over other people (which the wealthy certainly are), then they should account for that power with appropriate responsibility.
If you don't want to be a public figure, don't accumulate exorbitant amounts of wealth.
Do you think they will just… delete them? Because if so I’ve got a great time traveling hot tub to sell you