I would imagine a lot of it has to do with it'sits programming. Don't forget - at the end of the day it is a machine, and therefore governed by it'sits program.
It also comes from the future - so Skynet has a different observable world than the "present day" - which may factor in with the programming.
I would make an assumption that his runtime protocols would range something like this:
- Blend in as human
- Anyone who discovers you are not human, must be terminated
- Terminate primary target(s)
Of course this is wild speculation and conjecture.
Terminators are infiltration units - so the less attention they draw to themselves the better.