Kindle 的 personal documents 服务本来就是只供个人使用的。

Amazon 官网上与之相关的条款：

Your Use of the Kindle Personal Documents Service[1]
You may send personal documents to your supported devices for your personal, non-commercial use only. We may limit the volume or number of documents you may send, download, or store.

If you use Kindle Personal Documents to send documents to Kindles that are not registered to your account, our Distributor Terms of Use may apply to you. Please see the Kindle Personal Documents Distributor Terms of Use for more information.

其中明确指出，「如果要递送文档到非本人账户，需要遵守Kindle个人文档分发者协议[2]」。而这个 Distibutor Terms of Use 里也明确规定了该项服务不可商用：

No Commercial or Illegal Use
You may not charge directly or indirectly to distribute content via the Service.

「分发者不可就文档递送服务直接或间接收费」。豆瓣阅读虽然未对 Kindle 递送服务直接收费，却是满足了「间接收费」的，因为很显然，提供此项服务能促进 Kindle 用户在豆瓣阅读上的消费。而且亚马逊也声明了「保留单方面中止服务的权利」，所以真不能怪人家。

都是商业公司，总不能要求别人用自己的东西来帮你赚钱吧。不过还是期望将来豆瓣阅读能和亚马逊达成某种形式的合作，比如作为出版机构入驻 Kindle 商店，专卖个人作者的书籍。虽然好像不太现实。

[1] Amazon.com Help: Kindle Personal Documents Service
[2] Amazon.com Help: Kindle Personal Documents Distributor Terms of Use