It sounds to me more like the OP doesn't want to be peppered with 30-second bits of communication all day while he's trying to work on something else. You can't keep billing Client A while you're responding to an email from Client B, but if it's a 5-minute thing, what are you going to do, clock out and clock back in again all day?

So the communication from one client could be disruptive to other work, i.e. it could make a person lose money. If making a certain agreement with a client would cause me to lose money, it doesn't seem unreasonable to me that I would either charge for it or not agree to do it at all. In my experience, reasonable clients respect this.