File permissions allow finer granularity of access control.
Environment variables are visible to any user in the system.
strings /proc/*/environ
On Solaris/SunOS, you could use `pargs -e $PID`. And so on.
Having separate UIDs to run your processes A and B under shields either one from peeking at the other's environment, though. UNIX DAC is simple and powerful enough for MOST security concerns, I would argue.
This is completely false in any modern OS. You can only see environment variables of your own processes.