CL-USER
usually does not export anything. So it's useless to use it. That a package uses other packages, does not mean it exports anything. These are independent functionalities.
Since SBCL introduced an incompatibility to common practice (though not the standard), you have to specify the packages to use all the time - there is no default. Before that, there was a default (CLtL1 defined it that way). Now SBCL uses no package and this is allowed by the ANSI CL standard.
If you want a package with a use list like CL-USER, do this instead:
CL-USER 1 > (package-use-list "CL-USER")
(#<The COMMON-LISP package, 2/4 internal, 978/1024 external>
#<The HARLEQUIN-COMMON-LISP package, 3/4 internal, 271/512 external>
#<The LISPWORKS package, 67/128 internal, 228/256 external>)
CL-USER 2 > (make-package "FOO" :use (package-use-list "CL-USER"))
#<The FOO package, 0/16 internal, 0/16 external>
CL-USER 3 > (in-package "FOO")
#<The FOO package, 0/16 internal, 0/16 external>
FOO 4 > (+ 3 4)
7