It is very often useful to prevent people from accidentally violating encapsulation. It is usually a mistake to try to prevent people from intentionally violating encapsulation.
Caching to a global variable is very often extremely handy when writing unit tests. Nobody accidentally modifies a global variables in another package. Nobody on my team even does that intentionally in Production code.
And I've seen lots of really stupid things done to try to work around such attempts to prevent intentional violations of encapsulation. They are usually much worse than the behavior that was trying to be prevented.