I just got a brand new Denso MAF and tested it:
Looks like this is supposed to happen... so it's not a bad maf. It's that the maf is metal and pin #4 gets grounded through the pipe to the chasis.
NSFW is right, need to isolate the maf. I'll try layering some liquid electrical tape between the maf and the pipe and put some plastic washer on the screws that hold it