Regarding the license, they can define what constitutes authorized use of software to download materials and the circumvention of DRM is one gotcha.
“Limitations. Unless specifically indicated otherwise, you may not sell, rent, lease, distribute, broadcast, sublicense, or otherwise assign any rights to the Kindle Content or any portion of it to any third party, and you may not remove or modify any proprietary notices or labels on the Kindle Content. In addition, you may not attempt to bypass, modify, defeat, or otherwise circumvent any digital rights management system or other content protection or features used as part of the Service.”
They also can terminate your license at any time. In that case you would be obligated to cease using Kindle devices to view licensed content.
“Termination; Fraud. Your rights under this Agreement will automatically terminate if you fail to comply with any term of this Agreement. We may also terminate your right to use the Service at any time, including if we determine your use violates any term of this Agreement or involves any fraud or misuse of the Service. In case of such termination, you must cease all use of the Service, and Amazon may immediately revoke your access to the Service without refund of any fees. If we determine you fraudulently obtain or use any Tokens, we may revoke those Tokens and charge your credit card or other payment instrument for any products redeemed using those Tokens. Amazon's failure to insist upon or enforce your strict compliance with this Agreement will not constitute a waiver of any of its rights.”