To me it is kinda like Japan bombing Hawaii, and the US just saying, "it's alright, we are not going to do a damn thing about it."
Actually, it's more like Saudi Arabia bombing the World Trade center and the U.S. declaring war on Afghanistan.
Think about it: Nero -- a renegade Romulan -- destroys Vulcan and is narrowly prevented from destroying Earth. Barely a year later we see warhawk Admiral Marcus, who is trying to start a war with... The Klingons?
Rumor has it we might meet more Klingons in STXII, seems to me we need to deal with the Romulans.
We already dealt with the Romulans. Nero is dead, Kirk blew his ship to smithereens and flushed it down a black hole. Case closed. And the Romulan Empire at this time in history is technologically inferior to the Federation in almost every way; the Klingons remain, at this point, a far greater threat.