Who does, Kayak? The airline? I don't know about Kayak but that's not a thing with any airline contract of carriage that I'm aware of. In fact a lot of airlines explicitly say that their ticket does not guarantee particular equipment, seats, etc. Just that the airline will get you from the origin to the destination(s) in the cabin class you paid for.
Under what set of regulations? I only book flights on Airbus planes these days, but would like an extra insurance policy against switching in a Boeing.
