The described build error can be reproduced when the project B is WPF application. In this case the problem will be caused by the XAML compiler - it tries to load assemblies referenced by your dependencies (A) and fails if not successful.
To avoid the build error you need to add PostSharp reference to your B project as well. You will, of course, get the mentioned warning message then.
Starting with version 3.1, PostSharp does not add RequiresPostSharp.cs file to your project anymore, and performs validation inside msbuild script instead. Thus, you can safely disable PostSharp in the project B properties. This way you will avoid both error and warning messages.