They are power-passing, see NAS STD-9501 Satellite / Off Air Diplexer Power Passing (STD-9501) | STD-9501 [DIRECTV] | STD-9501 quad combiner off-air combiner STD-9501
75-ohm resistor caps should be used on all open SWM lines and splitter ports, so I guess you would need to terminate the diplexer.. But why would you want to use a diplexer without using the sat port?
Just curious, I don't even have an OTA antenna at the moment, but thought I'd go ahead and throw that question out there