This is a wonderful thing to see and I hope that more vendors will follow suit. Amit Elazari [1] has been doing some amazing work in this field advocating for legal safe harbours for security researchers. She posts regular reviews of security policies encouraging vendors to help protect security researchers using #legalbugbounty on Twitter. [2] In fact, it appears that Amit was responsible for some of the changes to Dropbox' security policy: https://twitter.com/d0nutptr/status/973322158351921152. Well done, Dropbox and Amit!
[1]: https://twitter.com/AmitElazari
[2]: https://twitter.com/hashtag/legalbugbounty