There's no reason to put this into the contract because the seller always has the right to offer "no discounts after survey". Maybe they've been "burned" before where they tried to make a deal and the buyer came back with a bunch of discount demands once the survey was completed. However, again, the seller has the right to look the buyer in the eye at that point and say no to any discount demands they produce.
As others have pointed out, you have the right to reject this deal after the survey, for any reason. And if you reject this deal, you are certainly able to offer the seller a new deal, based on the new information. It strikes me that this is just as silly as you writing a clause into the contract stating that you can reject the deal for any reason at all.
A long time ago I was moving from New Orleans to the Boston area, and I was selling the stuff I didn't want to bring with me. This included a cheap weight bench and set of weights. I had listed them for 50.00 knowing full well that I wanted to get rid of them, and they were at least worth that much. I figured that 50.00 was the bare minimum price to make the transaction worth my while. Any lower and it would have been easier to just donate or trash it.
The first call I got, (the minute the ad hit the street), was from someone who setup an appointment to see them that morning. When he arrived he starting pointing out how the bench wasn't a modern design, and that he could buy cast iron weights for .25 cents a pound, and on and on. I just waited him out and when he stopped talking I asked him, "So how much do you want to offer me for the set?". He said, "25.00 dollars". I told him, "Thanks, but I want 50.00. I've got your number and if I can't find someone to buy the set for 50.00, I'll call you back." He looked hurt that I hadn't agreed with him and sold him the set for 1/2 what I was asking. The very next guy that showed up within 60 seconds, asked me what I wanted, I said 50.00 dollars, and he said I'll take 'em.
Both parties should feel empowered in a deal, because they actually are.
Good Luck..