The definition of Ethical Behavior as defined in an entry from www.businessdictionary.com is:
Acting in ways consistent with what society and individuals typically think are good values.
Ethical behavior tends to be good for business and involves demonstrating respect for key moral principles that include honesty, fairness, equality, dignity, diversity and individual rights.
As you are discussing your decision and your action and in general our society has an expectation that people will honor the terms of the contracts that they enter into, then in order to act ethically you would likely need to either
- Destroy all of the copies of the business cases that you obtained under the license and not use them.
- Request an extension as user6726 suggests
- Purchase a new license to the material
- Borrow a copy of the material from a library that has license to lend the material to you.
If you entered into and agreed to a contract, whether implicitly as was the case here, or explicitly, then violating the terms of the contract without agreeing to a change of terms with your contract partner or receiving a legal judgement that the terms of the contract are illegal and therefore you are not obligated to follow them would be an unethical choice.
To further add to this would be the idea of fairness, as you mention with regard to the fact that the students that took the class were required to pay for the readings where as you received the material to use in the context of teaching them. The license was either paid for by the school, which means indirectly by the students or from a grant obtained by the instructor for the course, or was a comp from the publisher for the instructor selecting the material for the course in the first place. So while you could argue that your use is not really harming the copyright holder, the fairness argument to those who had to pay for the license, namely the students, calls into question the ethics of using them and strengths the case for not using the material.
That being said, I find the ethics of the publishers that chose these types of limited licenses highly questionable. When I have seen these types of licenses, they are generally for texts that get updated with great frequency, and the updates to most every chapter are at most a few sentences or a figure or two. It is part of a broader business model to make used textbooks less valuable and to force students to purchase the version of the text that the instructor is using. This is more the case where an instructor assigns homework problems and the problems are renumbered or parts of the problem are changed so that either the instructor needs to manage responses from multiple editions or they need to insist that everyone buy the new version. Another way to devalue used editions is to sell and get instructors to use online material which is licensed only to a single user and expires with the end of the course.
The problem is that two wrongs do not make a right. So just because the publishers are behaving questionably does not mean that it gives license to act unethically in response. Can you do it and live with yourself? Sure. But that is entirely your choice. And as you said, only you can answer that question for yourself. Strictly, in a legal framework you are breaking the terms of your contract and their copyright. And as a few of the commenters above stated, if you were to then use that material for new work produced, you could likely find yourself in trouble not only with the copyright holder, but also your institution if you published based on material you did not expressly have license to access.