Why is acidic cooking problematic for cast iron pans but not BBQ plates? I've recently discovered cast iron cookware and I'm a little bit in love! I have a couple of questions for experienced cast iron users. 

I've done lots of reading and it looks like acidic cooking isn't great for cast iron. You can do a bit in a well seasoned pan, but too much will affect the seasoning and/or affect the flavour of the food. I confirmed this the hard way when I forgot a marinade had lemon juice in it and totally destroyed the patina I'd built up.

I do lots of acidic cooking, as I love tomatoes and lemons!

So my question is, why are cast iron BBQ plates able to handle anything, including acidic marinades and being left out in the weather. What's the difference?
Iron is a very reactive metal, responding to many different influences. Heat, acidity, moisture, even the oxygen content of its environs cause reactions.  Barbecue plates are manufactured with enough other metals (zinc, aluminium, etc.) and via a tempering process that keeps the metal chemically stable in reactive environments. Cast iron is usually pure iron, and it just follows its nature when provided with a reactive environment.