People are getting arrested for all sorts of excuses. The Cop City protestor with the GrapheneOS phone was detained for "a routine search for child porn" - not something they routinely search for, and some people think they were going to plant some if they could get into the phone.

What I’m saying is that you are correct. No country can deny entry to their own citizens.

They can, and frequently do, arrest you the second they admit you.

What for? Whatever they make up.

In the US they can also keep all your devices indefinitely.