Generally considered predatory/unfair business practice.

You can hire anybody if you need them, and consider all applicants equally. But if you have 10,000 apps on file, hire 500 pilots, all of them from a specific smaller competitor even though many other apps are equal or superior to the 500 hired, that competitor might have a case. Worse if your recruiting obviously targets that specific company when there are many other good applicants at many other companies. Worse also if you offer unusual bonuses or the like.

It would be possible to damage a competitor legally by hiring many of their employees as long as the process was not specifically targeted at that employer. OK to damage the competitor as a second order effect, but the primary purpose has to be legit hiring, and the process has to be even for all applicants. Actually you could legally target a specific competitor IF you need their employees and IF they are the only source of those employees. That last would never apply to pilots, but might apply to personal in skilled niche technical fields (such as AI research). Non-disclosure and non-compete agreements generally mitigate that sort of behavior.

Not sure about specific laws, but I know there have been lawsuits.

"Systematic inducement of multiple employees of a single company to leave their present employment is unlawful when the purpose is to destroy a competitor or an integral segment of its business, rather than obtain the services of skilled employees. The very fact that a company targets its recruitment efforts at a single competitor suggests an improper motive, since one company rarely has a monopoly on skilled workers. Evidence that the loss of key personnel will harm the company, and that its rival desired to drive them out of business is important proof in these cases."