It is not 'apparently legal'. Ars' writers did an investigation, including talking to some independent attorneys, and the summary is that what they're doing is on shaky ground and likely to be ruled illegal. They are simply banking on authors not suing.
The authors are welcome to sue when the civil courts reopen in California, which may be a few months in the future, and already something of a fait accompli. It’s kind of like The Purge out here, albeit with felonies still illegal but most laws unenforceable and people sheltering inside in fear.