You should take on account that PDP-11 was pretty limited. You can't afford "wasting" memory on counters (for strings bigger that 255 chars) on an computer that

You should take on account that PDP-11 was pretty limited. You can't afford "wasting" memory on counters (for strings bigger that 255 chars) on an computer that handled few MiB at best, when you can use the null character trick, plus being efficient on these machine. Also, I think that not was motivation to make it secure on something that was initially a pet project on a unused machine.