It's not just Weblogic, some other vendors do it as well. Though I do not believe it is a standard. As to why, likely a design decision chosen from several possibilites. You as a developer have the same option. Use it for shared libs or put your shared libs some other place. Make your decisions based on what makes sense for a given problem and project.
as a matter of interest, if you place a shared JAR (say abc.jar) in the EAR APP-INF\lib directory and you want an EJB to use it, would you have to include the lib folder in the manifest file? i.e. should the manifest entry be lib/abc.jar or just abc.jar ?