why shouldn't that also be "heads-in" if you are so rated?
Because, you have a general duty to "see and avoid" when operating in VMC, regardless of the flight rules you claim to be operating under.
If you intend to simulate instrument conditions, either by wearing foggles, putting up screens, or simply by looking hard at the panel (not a great way to pick up experience BTW), then you are required by law to carry a lookout.
2D