I think you are talking about different things. I have absolutely decompiled obfuscated third-party Java libraries, fiddled with a few variables or method signatures, and recompiled + used them.
JVM security has nothing to do with this.
JVM security has nothing to do with this.
"Java-style stack inspection can restrict user or library code to deny access at runtime to unauthorized methods.",
which doesn't seem true to me, if the user has access to the library binaries. It's very possible for the user to just patch the library and use it however they want. It's possible (although I'm not especially convinced) that you can prevent the user from reflectively doing this at runtime, but that's not the only way to access unauthorized methods.
Could you explain what gave you the impression that this was about licensing schemes? That was definitely not my intention.