Is it because anti viurses runs in 'kernel mode' or do they have a special ACL?
I dind'nt find any thing on Google....
If you would have been able to terminate AV process, then virus process also would have been doing that, thus bypassing AV security.
Consider that the AV program has multiple executables running.
You can easily have each executable monitor each other to see if it's been terminated (not by the AV program, of course) and then relaunch the executable that has been terminated.