Even in the absence of an explicit rule, I think Martin is largely on the right side of norms here.

• Cara is a guest of Alpha Romeo in a promotional capacity. She received the access she received BECAUSE of her celebrity status, and with the understanding (formal or informal) that her visit would be used to promote Alpha Romeo and F1 via association.
• The grid is a restricted area where multiple live media crews have announced their presence in advance, and where it is known that opportunistic interviews are conducted. Martin isn't showing up in somebody's back-yard birthday party and avoiding him is the easiest thing in the world. This would be a different story if he was lurking in the parking lot, outside hotels, or sneaking into team hospitality suites. If you don't want to talk to Martin, don't go on the grid and he won't bother you.
• The nature of live media is such that, in snubbing the media crew, you're transitively snubbing their audience as well. There's no opportunity to not publish the story if the interview request doesn't pan out, the story of the the interview request not panning out is published live as it happens. We're riding on Martin's shoulders while he gets rebuffed, and it's happening to us too. Doing this to fans makes you a lousy promotional partner.

Because of the above conditions, IMO... you know what you sign up for when you enter the grid. It's no different than a red-carpet event or any other media/press event. If you don't want to interact with the press, no one is forcing you to go. If you go, and then get huffy about media wanting access to you, you suck as a promotional partner and teams should not invite you back (and should have clarified this beforehand and not have invited you in the first place if that's how you felt).

As to Martin's sass, I'm here for it. There's one place in the world where he's justified in shit-talking anyone who snubs him and I'd be disappointed if he didn't.