Now, as mentioned, in business other parties can also use their legal rights to bring about negotiations, but let's not depict Tesla as horrible for simply staying within the law...
What you want is much closer to mob rule, as seen by societies using the law for horrible purposes. Narrow legalistic views allow for the most horrible atrocities.
Everyone always makes demands that go far above the law. Your legal demand to those around you is "don't murder me, vandalize my things and steal the remains". Your full demand also involves amongst others respectful treatment and consideration, and you don't socialize with those you who treat your poorly even though such treatment was perfectly legal.
The law sets the bar for criminal liability. Society as a whole and its individuals freely set the bar for cooperation. A unionized work force is a social group that has formalized in advance the bar to cooperate with them.
You can absolutely expect people to do more than just obey the law. Everybody does. Tesla does not, and that may lead Tesla's employees to use their perfectly legal right to strike. And by your standards, you can't demand that they don't strike, because it's legal.
But it's a shitty basis for a relationship. If Tesla wants its workers to do good work for them, they need to listen to their workers.
Sure you can — "sign this contract to get this result" — just so long as you yourself aren't breaking the law in making the demand.
That said, we already have determined that generally just "sticking to the law" isn't a great defense in all situations anyway.