You answered to your qesution but I just want to share my experience with you. You're right, you can do that, but if you don't have at least one free auto-renewal subscription Apple will reject your app.
They will let you do your own subscription inside your magazine like those giant companies that have zillion types of subscription across all the devices, but they won't let you only use newsstand kit to do that.
You should have at least a free subscription button for your users, and if it is not free, you should do the other one which you set the price and time for it.
Because your content is either free, so no point of using third-party solution for subscription and just put the free button there, or it's not. Therefore, Apple thinks about its 30% revenue from your selling and if you sell outside and take advantage of newsstand, Apple won't let you do it.
Take a look at this link and definitely those two videos. It explains everything in it:
https://developer.apple.com/devcenter/ios/newsstand/