Because it's a means of expressing themselves and exercising their first amendment rights.
Let's see what a court has to say about it.
It's weird.
For the sake of the argument, let's stipulate that the app is indeed a government sponsored spyware, or at least a means to control and influence user's opinions... posing a real undue influence of a foreign power.
So can the app be bifurcated from the speech? Banning the app is not necessarily banning the speech. I just checked. It can be accessed over the web without the app... even on my phone through a browser. Of course I didn't log on. but it seems operational through html... Less full time tracking and such, I suppose.
Stipulate further, that there is a real denial of speech...
It's a monkey knot either way.
I honestly don't know how to weight it.
I tend to fall toward natl' security if the baby must be cut in half I guess... There are other means aplenty to exercise speech, and in this case it is not a palpable 'speech in dissent' within the American town square (which would be the speech most likely to injure liberty, being denied...)