A userland process can view/edit the memory of another userland process. Not so with kernel processes.

Viewing/editing of another userland process is usually done using ptrace(2). Yama Linux security module prevents that unless you are root to disable it. It is active by default on Ubuntu.

Also reading /proc/nnnn/mem does not work even for your own processes and even though file node protections seem to allow it, not sure where that security enhancement comes from.