I won't assert to know Tesla's motivation for open patents—and there is no doubt some goodwill involved—but do consider that it is not absent strategic advantage.

First and foremost, it's a form of deterrence. The presumption is that others will utilize Tesla's patents to compete, which in turn guarantees that said competitor can't sue Tesla for infringing on their patents. (Such activity is denied in the open patent promise.) It's kind of an informal, de facto patent sharing.

Depending on your opinion of mutually assured destruction, this is either a good thing or a bad thing. Good, perhaps, because of accelerated progress. Bad, perhaps, because this advantage is imbalanced towards Tesla. (They get to use any other company's patents—those that also use Tesla's—but those other companies do not by default get to use other competitors' patents.)

I mention all this free of ethical opinion. Just want to clarify that it is almost certainly not just a simple act of goodwill. Despite this, we can all—at least for now—benefit from such generosity.