"Otherwise there is no scenario where inflicting what you term a "brief disruption" (but what they'd likely describe as an awful, painful and expensive) to thousands of innocent customers is justified. Customer costs could easily be millions, if 1,000 customers each spend over ten hours of worker time dealing with that disruption."
That same logic would make it unjustified for them to be using such a small, unproven service to begin with. I mean, the first thing I do for mission-critical stuff is to see if the hardware, software, or service has supported long periods of uptime with easy maintenance and security patches. Also, has anything really bad like preventable breaches happened? And how are the servers configured by default?
Further, if you're worried about downtime so much, you have two providers in a setup with replication plus failover. The only people responsible for service going down and data destroyed completely in a world where basic HA is cheap are the customers. They should assume some shit could happen. They should mitigate it if it matters. Those that didn't took the risk willingly. Backups in particular are also really cheap these days.
"If the employer did something illegal, report them to the authorities. "
People have reported all kinds of big companies to the authorities for breaking the law. Goldman Sachs nearly destroyed the financial system. They got criminal immunity + $1 trillion from the government whose Treasury was run by their ex-CEO who profited off that activity. They and most of the rest like them still in business mostly without anyone doing time on the top. What's your next move for punishment if law doesn't care or is receiving bribes (esp Congress)?
"If you can't do any of those things because what the company did wasn't wrong or illegal"
Wrong and illegal are two different things. Slavery was legal but wrong. Locking child workers in buildings that might catch fire to force overtime was horribly wrong but legal. Civil forfeiture... taking an innocent person's money or property w/out charges... is wrong but legal. All kinds of abuse of employees, esp regarding promotions or references, is harmful to all but people on top and legal.
You're clear that no illegal action should be taken in response to a wrong. I'm guessing you oppose the underground railroads that freed slaves since they were illegal. You would have griped about it at best while all the harm continued to those people if working within the legal framework.