Indeed, the oil filter does not determine system oil pressure. There is usually something of a regulator on the output side of the oil pump. Generally and ball & spring affair in the main oil gallery.
See Bella749's answer and then also consider that the filter has FAR less resistance to oil flow than the places that the oil eventually goes to....Please correct me if i am wrong.
Oil pump>oil filter>engine parts.....and back to the oil pump.
But, any resistance in the system can affect the flow and pressure. In this case if the oil pump pumps the oil into the oil filter and the filter has less resistance, then the oil pressure must drop upstream regardless of any other resistance before or after the resistance(high flow oil filter).