The problem is one of false positives and prior probabilities. (also, it's westboro and wbc).
There's like 40 people in the wbc. Even if you have a 100% false positive rate and just include everybody, that's 40 people. Plus, if we know they're bombing things we know who they are. We don't know who potential attackers on the US are, besides the counterfactual media narrative that they may be Brown.
Relatedly.
As a white dude, I often assert my rights to avoid the pornoscanner and opt out. Once in a while get sent to secondary screening, because my shaving soap seems to have a similar gas chromatography spectrum to an explosive. They swipe me down, swipe down the soap container, and then the machine turns red and beeps. Further investigation ensues, and I am released.
They never treat me as a potential terrorist. That's probably because they know these machines are pretty terrible, and that the false positive rate is huuuge. But if I were? If this was indeed a dangerous substance and I a dangerous human? Well, the first time, they left me unsupervised in the same cubicle where they were storing their walkietalkies on a charging rack. Unsupervised, beyond just "stay there". Seriously.
They know it's a false positive and it's treated very lightly - again, I'm a White dude with no obvious marginalizing markers. So yeah.
If by "tell", you're talking about pulling aside everyone who "seems Muslim" (obligatory discussion of how Sikhs are often the target of hate crime from people who think they are Muslim because of their headwear here), you're pulling aside just under 1/3rd of the worlds' population. You can't use that information because it's literally useless.
The further complication here is that white nationalist terrorists are just as dangerous as non-white ones, but this administration is focusing on non-white terrorism to the exclusion of everything else.