It's extremely sad to receive emails from the White House signed by our president or other senior officials that consistently make this error. A pair of hyphens "--" is not an appropriate substitute for an em dash "—" according to every major style guide and writing resource center. A simple Google query for "difference between hyphen and dash" will return almost 100,000 results that insist these are not interchangeable, including results from notable websites such as the Chicago Manual of Style and the Purdue Online Writing Lab.
Please make it right and teach your writers to write right!