Why does it matter how he joins? He joined because Luffy had the power to save his ass from the marines while Zoro couldn't save himself. Did he join and remained as Luffy's underling or not? That's all that matters. Its not like he was still being forced to stay, he could leave whenever he wanted.
That and Zoro said his captain should be stronger in the fishman island after seeing Luffy knocks out 50.000 men, Zoro himself expects his captain should be stronger, and tell that to the people who calls him bitch if you are serious not to me lol.