When a piece of software explicitly requires that their open source license acknowledgments be included in derivative software (software that uses the open source software), is this a legally enforceable requirement?
According to this blog post it is, but let's look at an example. PayPal have an open source project, card.io, that third-party mobile app developers can use for scanning credit cards in their apps. They state:
Add card.io's open source license acknowledgments to your app's acknowledgments.
However, when I downloaded apps that use this project (Uber, PayPay itself, TaskRabbit, EAT24, grubHub, etc.), in some cases I found that they had a section on software licenses, however I did not find the license acknowledgments for card.io inside any of the apps (Uber is a good example of this).
What are the legal ramifications of not including the license acknowledgments in a visible part of the app (let's say it's a banking app here)?