Why? There's a legal concept called "promissory estoppel". In a nutshell, it means that if I make you a promise and you, in good faith, depend upon that promise and build your business on it, and I knew or should have known that you were going to do so, then I can't later change my mind, withdraw my promise and sue you for doing what I said you could do.
Yes, it appears that "promissory estoppel" can create an implied contract, but it's not at all clear to me that a press release meets the standard. More to the point, "promissory estoppel" appears to be situational - not absolute. And you can bet Tesla's lawyers know that - hence the weaseling "in good faith" and "in the spirit of open source" (without actually specifying what that means or which license applies).
In the end, I suspect the OP is correct - without a signed and sealed contract or other appropriate and valid legal instrument... nobody with any sense and/or a half decent lawyer will depend on a press release and Tesla/Musk's continued good intentions as a legal defense against infringement. Particularly this applies to the big automakers or any real money investing in a smaller automaker. This is an impressive publicity stunt, and it will impress the impressionable and the fanboys, but without the proper legal backing that's all it is. The fanboys will use this press release to bitch and complain about the Big Automakers - and will never clearly understand how neatly they've been played. (To my mind, given that Tesla and SpaceX are still both essentially startups skating on thin financial ice, it's an open question as to Musk's acumen as a businessman. But one thing there's no question about to me, in the last few years Musk has emerged as a master manipulator of public opinion.)
Who needs to compete when you have lobbyists?
Rest assured, the second they perceive Google as a real threat, there will be a bevy of laws passed to obstruct this sort of deal. Just like they've already passed laws in 30 states banning municipal broadband.
Google has lobbyists, too.
I don't know of any challenges, but the principle in question seems nearly identical to the copyleft notion underlying the GPL -- a notion that went untested in court for a very long time because, basically, every attorney that looked at it decided it wasn't worth fighting. As for whether an assignee is obligated to honor the license, I can't see how they could possibly avoid it, particularly if the license states that it's irrevocable (excepting its exceptions). I'm not aware of any purchaser of a patent who has successfully argued that they can revoke a licensing commitment to a standards body, either. It seems to me that the precedent is rather firmly established.
The price of scopes goes up rapidly with bandwidth. 20MHz is cheap. 100MHz is more expensive. 1GHz and up is very expensive. Tektronix sells a 33GHz scope starting at $30K. That's the first question you have to ask. For kids doing Arduino-level elecronics, 20MHz is fine. None of the I/O goes faster than that. (If you want to look at Ethernet or digital video signals, you need far more bandwith, more than you can afford. Fortunately, today that stuff mostly works.
(Back in the 1990s, I was trying to build a LIDAR unit for a robot. The parts aren't expensive. The problem is that, when it didn't work right, I needed a high-bandwidth scope I didn't have to find out why.)
Tell that to all the people who based their software off of Java, who used the API in good faith.
None of whom have been successfully sued. Oracle tried to sue Google, not based on using the API, but on re-implementing Java. And they lost.
The plaintiffs would have to demonstrate how the company lost value by them doing that.
Also, if the articles of incorporation or the IPO documentation includes a statement indicating that increasing the use of electric vehicles is a corporate goal, alongside of, or even more important than, making money, then this move could be perfectly aligned with the stated corporate goals which the investors bought into, even if it can be proven that this move decreased share value.
There is a middle ground. They could issue a zero-cost, binding, globally-applicable patent license with an exception that withdraws the license from anyone who sues them. This is actually pretty common, and I think it would be a much better choice than placing the patents in the public domain.
I expect something like that will be forthcoming. So far all we have is a blog post; I imagine the more substantive version from the legal department is in progress.
Facebook is notorious for making the "opt out" icons invisible until you mouse over them. Opting out in Facebook is like playing one of those old Flash games where you mouse around the screen, trying to find the hot spot that will accept a click. Are they going to do that again?
What do you mean by Large Scale Deployment?
IPv6 being used to route a significant percentage of Internet traffic.
Target shooting, competitive shooting, hunting and self-defense are other legal uses of handguns.
Imitation is the sincerest form of plagarism.