Big assigments makes most shells crash?

[Matthieu PARISOT]

When you try in bash 2.04/2.05 and zsh 3.0 some commands likes this:

mat@home:~$ a=$(perl -e 'print "A"x100000000)

the shell sometimes crashes sayin' 
bash: "xrealloc: cannot reallocate xxxx bytes ( 0 bytes allocated)"
zsh: "fatal error: out of memory"

The strange of the thing is that if you put a really big string, bash sometimes 
detect it, hang cleanly, say 'out of memory', and give you the hand.
I've noticed that csh is more radical on this feature : it tells you immediatly 
if the string is too long or not (but the limit seems to be low)

So why sometimes it just fails, while sometimes it crashes ?