That's not totally accurate... If you use our SaaS Chart Studio product then yes, but otherwise (i.e. in 'offline' mode) you can use our Python, R and Javascript libraries as much as you like: they're MIT licensed.
I'd like to add that Dash helped me grok React far quicker than I'd expected. I code in a lot of React now and I'm only here because of Dash. Thank you for your hard work!