Most rescues I've worked with just have those clauses in there so they have a little leverage in case they find out the dog is in a really bad situation. Even if they know the clause is likely to be unenforceable (which at least some do), it lets them go up and say, "You can surrender the dog now or we can take it to court and we can sue you for additional costs like attorney's fees as well." That's enough to get most abusers to surrender their dogs, since they're not exactly attached to them.

I've volunteered with rescues for years and only saw those clauses used twice (two different rescues). One was a guy chaining a dog out 24/7 (which itself was a violation of our adoption contract, but honestly we probably wouldn't have acted on just that), without shade and often without water, in an extremely hot climate where unfortunately that kind of treatment was common so animal control would have taken forever to act. The other was an adopter whose cousin called us and told us that the adopter was starving the dog and it was covered in untreated sores, things like that. So both pretty extreme situations.

The first situation, threatening the guy with a lawsuit for breach of contract was enough to get him to surrender the dog to us on the spot. The second one was unfounded; we knocked on the door and the happy, healthy dog greeted us with her adopter. That one we didn't even ask to go inside or anything, just apologized for the trouble (the owner did graciously invite us inside; apparently his cousin did this kind of thing a lot so the poor guy was used to it).