Silly questions. Both poison and freeze drones do no damage at all, only inflict an 'innofensive' statuses. But a shield gets damage from them....
How the hell does that work?
Fist guess is that when a status is supposed to trigger but meets a shield, it translates into a damage tick.
But with that in mind, how does a shield work against a mosnter that has a CHANCE to inflict a status then?
Say a fire zombie, they wont always burn you with their swings. How would the shield react? Does it always take the same exact damage or does it suffer extra damage from time to time when an attack would have caused fire?
This is how shields work in general. To a shield, both damage and status are forms of "damage", in that they lower the shield's health, until it "dies" (breaks). A shield blocks all damage and status, until it breaks; once broken, it blocks nothing at all, until it regenerates a bit. Have a look at the wiki articles "Shield" and "Shieldbearer Guide".
Edit: I don't know about your chances-of-fire example. I suspect that the shield has the same chances as an unshielded knight would, but that's just a guess based on simplicity of programming. Certainly, as you stand in a zombie's breath with your shield up, you can see the "hits" that the shield is taking.