Unfortunately "adult" does not have a universal definition in this context.
However I'm with you... I'd never work somewhere again (circumstances permitting) that did not offer me the flexibility I need to perform well. Sometimes that means I take a Monday and do nothing because I'm not recharged and my brain isn't being productive. Sure I could try to force something out, but honestly that doesn't benefit me or the company. My output would be poorly thought through, and I'll grow resentful. The tradeoff is, when I'm in the zone on something, it tends to consume me a bit and I'll gladly work until I'm done with it, nights or weekends be damned, because it's interesting.
Knowledge work is not a "show up and punch the clock" gig. Brains have off days and inconsistent output. When the quality of the thinking matters, and impactful output scales way beyond the cost of the individual employee, it pays for employers to be flexible.