Why is lc_messages superuser only?

Looking around the lc_messages stuff a bit, I notice it's set to
superuser-only. 

I do use
ALTER USER joe SET lc_messages='sv_SE'

now and then to change the language for a user. And I see it's also
possible to use it on a database level by doing
ALTER DATABASE postgres SET lc_messages='sv_SE'

(user overriding database overriding system default, as expected)

However, it can also be useful for the user to be able to change his own
session, and this only works if you are superuser.

Is there a reason for this?

//Magnus