From the manpage
w displays information about the users currently on the
machine, and their processes
To display the users' processes, it goes through all processes running on the machine. Let's try this:
$ strace -o w.trace w | grep whatever
Inside the trace we find lines like these (on a Linux system):
open("/proc/8286/cmdline", O_RDONLY) = 4
read(4, "grep\0whatever\0", 2047) = 14
w explicitly going through
/proc and looking at the command lines of all processes (and other things, not shown). It finds the
grep that runs parallel to it and that's what
strace sees it do. The pipe has nothing to do with it, other than starting both processes at the same time. In a way, it is similar to
ps | grep seeing the grep itself.
who and most other commands don't need the information about the processes, and don't go looking, so you don't see the same when tracing them.