1. Application developers frequently don't know how the libraries they're using work, and often don't even know what libraries they're using because they're transitive dependencies (even a version update of a library can pull in a new transitive dependency). If a library imposes any kind of risk on the application, the application has to acknowledge that it accepts it.

2. The last word is given to the application, and the point is that libraries must not make decisions that have a global impact on behalf of the application. That's why an executable JAR, i.e. an application, can grant such permission in its manifest but library JARs cannot.

See more here: https://openjdk.org/jeps/8305968, https://openjdk.org/jeps/8307341