How to respond to publishers when they ask for rules?
Send the rules. 


Fear of having your idea copied/stolen is a uniquely amateur mentality. Daniel Solis (designer) and Gil Hova (publisher who references other publishers) cover the topic well in their articles about this very topic. Additionally, there is this entry on BGG. They all conclude that you should not worry about someone stealing your idea. 
The company asked for the rules. Do you suspect they won't act in good faith? If so, the rules issue is moot. Otherwise, please know that this is standard practice. Earlier this week, I made appointments with about 20 designers for Gencon - and declined several others - after looking at a combination of rules and sell sheets. Let the professionals see your game for what it is!