They did, several times. This has been going on, and off, for years.
It was just embarassing to the American administration and was not publicized.
"According to media sources, in August 2006, both Turkey and Iran carried out artillery attacks in the northern frontier region of Iraq in order to weaken Kurdish rebel forces, in particular, the Kurdistan Workers' Party (PKK) (The Economist 31 Aug. 2006; The Guardian 18 Aug. 2006)."
http://www.unhcr.org/refworld/country,,IRBC,,IRQ,4562d8cf2,45f1475730,0.html
