I feel for you Scarlet (and all those here in the same situation). (((Hugs)))
But with new and conflicting information coming out about smoking all the time, I'm afraid I can't blame parents for erring (if that's what it is) on the side of caution. True, they can't control all the places where their kids might pick up "third hand smoke." But that might be why they are all the more cautious about those places they know about and can keep their kids' away from. Sad when this impacts GPs, but can't be helped, I guess.
I understand your not wanting to "badger" DH. Has he tried to quit though? Given your compromised immune system, I'd think he would. Or at least confine his smoking to the outdoors.
Anyhow, sure, some of us grew up with smokers and weren't hurt by it. But many developed asthma and other health conditions due to their parents' smoking, we just didn't know it. And sure, some parents let their kids visit the homes of relatives who smoke. But obviously, that doesn't mean other parents "should" or "have to."
Star, it does sound as if your DS and DIL were making excuses ( unless there were other issues that you didn't realize in addition to the smoking). But that doesn't mean Scarlet's are.
Scarlet, it will take time, but I think you need to learn to accept this disappointment and just enjoy the visits you have with DS and family elsewhere. Best to you!