Idk if I'd go as far as to say it's a dick move...I think it's fair to say that we are at the airport (regardless of inside or right outside), so the average person, especially if they either don't fly a lot or don't use the curbside services, wouldn't know the common practice of tipping in this exact situation - since I do not believe that it is remotely close to common for people to tip inside the airport at the check-in area.

To my knowledge, I don't even think there is a sign or anything near curbside check-in giving a heads up about "gratuities are appreciated" or similar wording - please let me know if I'm in the wrong

Especially with the point made earlier that it is the same service essentially as inside as far as the customer is concerned, so again, I don't think the average person can be blamed for making that mistake.

And, if I didn't have cash on me for whatever reason to tip in that situation, the dick move would be for them to give me my bags back and uncheck me - after all it is a gratuity and not a service fee.