tldr: good faith = don't be a dick
Disclaimer: IANAL, and I am not particularly familiar with equity law, which (promissory) estoppel falls under.
In contract law, acting in bad faith refers to following the literal wording of the contract while taking actions that would deprive the other party of their benefits. e.g. you lease a car, but do not provide the car key. In some jurisdictions there is an implied duty of good faith in contracts, while in others explicit terms are required.
Now, in this case there is no contract because there is no quid pro quo relationship between Tesla and the licensees (the legal term is consideration). Instead it is enforceable by promissory estoppel, which essentially means Tesla can't sue someone who relied on their statements.
Since Tesla isn't explicitly receiving anything in return for the license, it's unclear what an action in bad faith could deprive them of.
One argument would be that the benefit Tesla accrues from this is that other companies will (hopefully) build infrastructure. It's entirely possible that they may choose to do so using Tesla's patents but with incompatible, DRM'd connectors. I suspect this is what the good faith requirement is intended to prevent. (Non-DRM'd connectors would be more of a grey area, since they could argue that it was used simply because their technology is better.) In other words, the other company would not be permitted to prevent Tesla from supporting their charging stations if they used the patents in question.