[Hitman]

There are two reasons why the Kaleun would conduct an attack from the observation scope:

1.- As you said, because it was all a "show" for the purposes of filming, and the control room was a better location for the filming (The conning tower is too small to allow one extra operator with a big camera). Add to that, that the fixed heigth attack periscope was a technology that allies didn't master at that time (The US had attempted to create such scopes in the Barracuda, Bas & Bonita class subs, but the results were impractical), and didn't know the germans had. So it is logic that they wanted to keep it secret.

2.- Because it was an attack in low light conditions, not low enough to attack on surface, but too low for using the attack scope (Which had a thinner head, and as such, a much smaller light admision, resulting in a darker view, as if wearing sunglasses).

Kaleuns would conduct an attack as follows:

Daytime: Submerged, using Attack periscope
Twilight/clear night: Submerged, using Observation periscope
Dark night: Surfaced, using the UZO