Exactly my thought process. Why's he doing that? It's either to stop what the attacker believes is a promising attack, to delay the release by the GK to slow the game down, or because he's just being a dick. Or to put it in referee speak, it's either C1 (SPA), C4 or C1 (general USB).
I'm not saying you have to pull out a card on a first offence, or on a token gesture that doesn't actually influence the GK. But if the temperature of the game demands it, I think you can always find a reason to justify the card - and you want to be having a word with the player at the next opportunity regardless.