Apparently he recruited them while working for Tesla, which is immoral whether or not it's illegal: if you're in senior management, people feel like their job is on the line when they say no to you. That's why you're not allowed to ask them things unrelated to the company's interest where they might want to say no, such as asking them on a date. The same power dynamics are at play when asking someone to jump ship to your new venture - while you remain employed at the old one, you have power over their reviews and their working relationship with the rest of the company.