why desktop apps are able to kill my freebsd box?

[Michael]

Why a faulty desktop application run as unprivileged user is able to 
crash my system?

I mean, I know programs have bugs and sometimes they lead to crashes. 
I'm fine with that. But why a crashing program (for example firefox or 
banshee) is able to kill the whole system?

And by 'crash' or 'kill' i mean that for whatever reason the system is 
frozen and doesn't reply to anything but a hard reset.

It just doesn't make much sense so I'm assuming that I must be doing 
something wrong. Where and what kind of restrictions should I set up to 
make sure that a buggy unprivileged program cannot freeze my box?

Thank you in advance. Michael.