That's because they installed a switch. What you have is not a switch, but a sending unit for a gauge. It has a basic rheostat inside. The more pressure applied, the more of a connection is given and the gauge moves up. It should be an open connection when checked as you have.
Also, I'll add this since it could be relevant. I just recently went through THREE bad brand new oil pressure switches (for a light) for my slant six. All Standard Ignition products. Part number PS16. First one never worked. Second one worked for a short time and failed leaving the light on. Third one worked for two days and failed where the light never came on. So.....I hawked eBay and got an NOS Chrysler pressure switch and all is well. I never thought I'd say this but Standard Ignition SUCKS. Thanks, China.
