They're not going anywhere, they've gone to ground!
In each case, I was able to direct them to the main rulebook page 24, "Going to ground" section, to the last paragraph that very explicitly states that the unit is affected normally by enemy actions, giving the example of taking morale tests as normal. When I showed them the section in the rulebook, they went along with the ruling, but begrudgingly with a comment close to "we don't play it that way" or "that's not the way it used to be."
My reaction to hearing "That's not the way it used to be."
I'm not going to claim that I haven't ever been wrong about a specifically written rule, but I just find it a weak cop-out to say something like "I don't play it that way" or "That's not the way it used to be." The times people have pointed out a mistake I have made in the rules, I check out the rules on that, admit my mistake and move on.
With a complex rules system like this, it can be easy to miss rules here and there. We're all human and can miss bits and pieces here and there. Also I can understand either if someone is brand new into the game, or 5th edition. Everyone deals with the transition between new rules editions , but it's been more than two years since 5th edition came out and regular players still cop out with excuses of old rules.
I don't even see the validity of the excuse of "that's not the way it used to be," because nobody has qualms with using the beneficial aspects of 5th edition (Going to Ground voluntarily for improved cover saves, etc). These people don't mind going to ground voluntarily to get an improved cover save, but as soon as the negative portion of the rule creeps up (affected normally by enemy actions), they put up the defense "we don't play it that way" or "that's not what the rules used to say."
"What do you mean that's not how the rules are? I hate these new rules"
They're picking and choosing which rules they want to follow for their own advantage, whether deliberately or sub-consciously (remembering rules they like and forgetting rules they don't like). I don't mind gaming groups that wish to "house rule" or play the game in the way that makes them happy, but when you're at a regional tournament that is going by the regular 40k rules, "house rules" don't fly.
At some point, we all need to take some personal responsibility to have a general understanding of the game rules and when presented with something that we're unaware or unfamiliar, look at it rationally and resolve it amiably.