I think, after talking to pyr8queen about it, it's because it's something show-biz people say to each other on their walkie-talkies. So screenwriters think it sounds cool and military, so they pretend that it is. So listen up, and prepare to copy; it's annoying and incorrect. Roger?
edit to add, on reflection, "How copy?" Is used to mean, "Do you understand?" but it's actually incorrect and not on the official list. It's a common mistake though that even military personnel make. Or at least used to - maybe I'm just too old school - my Army skills are all Twen-Cen after all.