Actually, it likely has more to do with the fact that the Loonie thing came up in Blaze of Glory, Eddington's last episode. Kenneth Marshall, who is American obviously had no reason to think the character was Canadian prior to that so had no reason to pronounce the letter differently than the American norm. It is ironic the character they chose to give a Canadian connection actually frequently had to speak a letter that is pronounced differently in the US and Canada (his dialogue does include many references to the DMZ).
I just kind of assumed that all pronunciation had become Americanized by the 24th century. Because not only does Eddington say the letter Z the American way, everybody says 'Lieutenant' the American way as well, including Picard, Reed and Eddington.