I don't know the answer to your questions, but it is simply the law unless the employer voluntarily recognizes the union. Obviously employers don't have much incentive for doing so.
source: https://www.nlrb.gov/rights-we-protect/whats-law/employees/i...
Why do you, what's the reasoning behind that kind of law? And to whom do they petition - does the company have to agree, or rather, does the employer have the right to forbid employees from forming agreements between themselves? It sounds counterintuitive when you only need 1 person to start a company, or 2 people to have a contract, and when employers have no other control over who employees meet with and little control over how employees spend money (e.g. 'work dress code').