There isn't. He didn't have to respond at all. However, he expressed, through his language, an unwillingness to negotiate the contract. That's why he ended up not negotiating the contract.
It's also why when people address me or my employer the way he did, I just don't engage at all.
Oh, so not only would you be impolite enough to send such a contract, when questioned on it you would also be so embarrassingly impolite to not even answer that you stand by your previous insult. Gotcha.
(1) it's possible that they didn't know the language could be used to hurt someone in that way. You won't give them the benefit of the doubt and instead would rather give that benefit to the person who was actually impolite.
(2) Again, as I said before, I don't agree with those terms. That doesn't mean I'd be an ass hole about it. To my benefit, I'd politely negotiate and have a much higher likelihood of negotiating the contract to my liking.
(3) If someone posed the same concerns to me in a polite and professional manner, I would go out of my way to correct the contract (hey boss person, this contract might be bad for developers!). On the other hand, if someone is an ass hole, they're going to an "ass hole" response.
Try it. Try being polite in a situation where you'd like to be an ass hole. You're going to be surprised at the end result.
It's also why when people address me or my employer the way he did, I just don't engage at all.