Do they have an infant/toddler?
With the pertussis outbreaks all over the country, I can see why they would be so strict if they had a baby or toddler in the house. And in a way, it could set up a double standard if you had older children as pertussis is dangerous for babies and toddlers more so than older children, teens or adults.
We had the "must be healthy" rule to visit when our youngest was an infant and into his toddlerhood years. Of course, if we got a cold, we would be sure and let our guests know so that they could make the decision whether or not to visit. Most in my family just said "what a little cold? No, we will still show up." Ha.
But, it was still their decision to make, not mine. And what your friend did would truly upset me.
Next time she invites you to her house, feel free to tell her "Oh, if anyone in your household is sick, please be sure to let me know as we don't like to visit sick people." Yes, it's blunt but she's blunt with you so if you hurt her freelings, she's just being a SS.
If you don't want to visit her anymore, just say "sorry, that won't be possible."
And if you do show up at her house and realize her children have the sniffles, it wouldn't be rude to leave. I've done it before. "Gosh, so sorry, I didn't realize little Bobby was sick." "Oh it's nothing." "Unfortunately, it can be for us so we will have to get together another time." Smile and leave. It's not a big deal if it's done in a "it's not a big deal" sorta way.