wrx A/C turned on by itself on defront setting?

Yes, it's normal.

Stupid, but normal.

Yes, normal and common on lots of cars, not just Subarus. The AC dehumidifies the blown air, the heater core heats it, and when it hits your windshield, it's hot, dry, and ready to suck up all that moisture. It can be more than you want/need tho, and some folks find it undesireable.

If you don't like this feature, it's pretty easy to change. Remove the center trim around radio/HVAC controls, and dismount the radio/sound unit. Looking down at the various HVAC gears & linkages, you'll see a small electric switch mounted on a white plastic gear on the passenger side, towards the back. The switch is mounted with one small phillips head screw. The switch is depressed when you rotate controls to Defrost position, and activates the AC. Remove the screw and tape the switch assembly back where it won't bother any moving parts, and the AC will no longer activate with Defrost. I put the screw in a small ziplock back and taped it up in the housing.

If you want to run the AC with Defrost, you can selectively activate it by pressing the AC button, whenever you like.

Yup.

Set to defog the A/C system tries to condenses water out of the air before blowing it on the cold windscreen where it would condense creating more of the fog you were trying to get rid off.

Then you've never had the experience of driving your Ford product in the fast lane, in bad weather, and switching on the defroster only to see your entire windshield go completely opaque from condensation.

Been there, done that, will take the Subaru approach any day.

Mine does the same thing. Except, the A/C light comes on no matter what position I have the fan switch in. Well, except for off!

BlueSTi "Scary-Fast"

That control position is not just directing air to the windshield, it also activates the AC to condense excess moisture out of the interior air.

Florian