I am uncertain how this letter changes the established case law regarding C Corps.
No corporation is required to "maximize shareholder value". In fact, they cannot be because there is no possible legal standard for what constitutes "maximizing value".
B Corporations have no special legal standing. It's just a certification created by a group that tries to push corporations towards responsibility (or, more cynically, provides virtue-signalling-as-a-service). Any corporation can adopt any or all of the principles set out for B Corporations, if the changes to their charter and by-laws are approved. It is the existence of such requirements in the corporate governance documents that provides the legal requirement, not a private certification.
You're correct that B Lab's certification has no special legal standing. A "benefit corporation", however, is a recognized legal entity in many US states [1]. Patagonia is both a certified B Lab B Corp and a private benefit corporation. I should have made this point clearer.
> No corporation is required to "maximize shareholder value"
The rise in benefit corporation legislation was in part caused by the 2010 eBay v. Newmark decision in Delaware, which included the statement, "Promoting, protecting, or pursuing nonstockholder considerations must lead at some point to value for stockholders." [2] Note that this is still a vague statement, not using "profit" or any specific timeframe. I know that this case is still hotly debated, and I don't know enough to comment on all its implications.
Outside of a legal framework, it can be argued that all corporate directors that are non-majority owners of their public companies do have a requirement to "maximize shareholder value", or, to radically oversimplify the process, they will be replaced by shareholder vote.
So I should drop the "legal" modifier to my comments, and just refer to "obligations" to the shareholders and "action" by the shareholders.
Again, I do not know what significance the recent letter has on any of this.
[1] https://en.wikipedia.org/wiki/Benefit_corporation [2] https://www.delawarelitigation.com/uploads/file/int51%281%29...