Some people are very protective of their e-mail addresses. When you collected those addresses, you stated, or implied, that you were collecting them for PTO purposes. Did you tell people that you would be sharing their e-mail address with the school?
I certainly do not find the next school's request for the e-mail address unreasonable. And I wouldn't mind if mine were passed along. But I cannot speak for other people, so I would have to say "sorry, but I can't provide that information."
On the other hand, if you told people that you would be providing their e-mail address to the school, then I would see no problem with providing it to the next school. After all, your school could ask for that information and then share it with another school.
Given that your principal doesn't want it done, I would suggest following that advice.
I was asked by one of the other schools secretaries if I would be able to give them a copy of the parent's e-mail addresses for students who will be leaving our school this year and go to their school next year. I feel they can get this information at registration time in the fall and do not feel I should release this information. I did speak to our Principal about the situation and they do not want me to send the list. Also, is this kind of like selling the e-mail addresses? I want to be sure I am doing the right thing and don't need parents or administrators getting on me about it.